[Bug]: validation tests fail due to subprocesses running from missing dir

[oruebel]

What happened?

When running python test.py from the main directory, I get errors from the validation test suite because it seems to try and run subprocesses from a subdirectory coverage that doesn't exist (see trace below)

Steps to Reproduce

Run ``python test.py`` from the root dir.

Traceback

=====================================================
2022-08-10 12:47:57,569 - INFO - running validation CLI tests
EEEEEE
======================================================================
ERROR: test_validate_file_cached (validation.test_validate.TestValidateScript)
Test that validating a file with cached spec against its cached namespace succeeds.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/oruebel/Devel/nwb/pynwb/tests/validation/test_validate.py", line 59, in test_validate_file_cached
    result = subprocess.run(["coverage", "run", "-p", "-m", "pynwb.validate",
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 505, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 1821, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'coverage'

======================================================================
ERROR: test_validate_file_cached_bad_ns (validation.test_validate.TestValidateScript)
Test that validating a file with cached spec against a specified, unknown namespace fails.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/oruebel/Devel/nwb/pynwb/tests/validation/test_validate.py", line 71, in test_validate_file_cached_bad_ns
    result = subprocess.run(["coverage", "run", "-p", "-m", "pynwb.validate",
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 505, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 1821, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'coverage'

======================================================================
ERROR: test_validate_file_cached_hdmf_common (validation.test_validate.TestValidateScript)
Test that validating a file with cached spec against the hdmf-common namespace fails.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/oruebel/Devel/nwb/pynwb/tests/validation/test_validate.py", line 84, in test_validate_file_cached_hdmf_common
    result = subprocess.run(["coverage", "run", "-p", "-m", "pynwb.validate", "tests/back_compat/1.1.2_nwbfile.nwb",
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 505, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 1821, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'coverage'

======================================================================
ERROR: test_validate_file_cached_ignore (validation.test_validate.TestValidateScript)
Test that validating a file with cached spec against the core namespace succeeds.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/oruebel/Devel/nwb/pynwb/tests/validation/test_validate.py", line 95, in test_validate_file_cached_ignore
    result = subprocess.run(["coverage", "run", "-p", "-m", "pynwb.validate", "tests/back_compat/1.1.2_nwbfile.nwb",
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 505, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 1821, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'coverage'

======================================================================
ERROR: test_validate_file_no_cache (validation.test_validate.TestValidateScript)
Test that validating a file with no cached spec against the core namespace succeeds.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/oruebel/Devel/nwb/pynwb/tests/validation/test_validate.py", line 24, in test_validate_file_no_cache
    result = subprocess.run(["coverage", "run", "-p", "-m", "pynwb.validate",
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 505, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 1821, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'coverage'

======================================================================
ERROR: test_validate_file_no_cache_bad_ns (validation.test_validate.TestValidateScript)
Test that validating a file with no cached spec against a specified, unknown namespace fails.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/oruebel/Devel/nwb/pynwb/tests/validation/test_validate.py", line 42, in test_validate_file_no_cache_bad_ns
    result = subprocess.run(["coverage", "run", "-p", "-m", "pynwb.validate", "tests/back_compat/1.0.2_nwbfile.nwb",
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 505, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/Users/oruebel/miniforge3/envs/py4nwb/lib/python3.9/subprocess.py", line 1821, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'coverage'

----------------------------------------------------------------------
Ran 11 tests in 0.175s

### Operating System

macOS

### Python Executable

Python

### Python Version

3.9

### Package Versions

_No response_

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/NeurodataWithoutBorders/pynwb/blob/dev/.github/CODE_OF_CONDUCT.rst)
- [X] Have you checked the [Contributing](https://github.com/NeurodataWithoutBorders/pynwb/blob/dev/docs/CONTRIBUTING.rst) document?
- [X] Have you ensured this bug was not already [reported](https://github.com/NeurodataWithoutBorders/pynwb/issues)?