Hide stack traces during test run

[mccalluc]

These stack traces show up during the test run: The tests still pass, but the noise is distracting:

test_bad_config_html_error_true (tests.test_app_runner.TestAppRunner) ... /Users/chuck/github/refinery-platform/heatmap-scatter-dash/context/app_runner.py:114: UserWarning:

Traceback (most recent call last):
  File "/Users/chuck/github/refinery-platform/heatmap-scatter-dash/context/app_runner.py", line 57, in main
    raise Exception('Either "demo" or "files" is required')
Exception: Either "demo" or "files" is required


ok
test_default_args (tests.test_app_runner.TestAppRunner) ... /Users/chuck/github/refinery-platform/heatmap-scatter-dash/context/app_runner.py:114: UserWarning:

Traceback (most recent call last):
  File "/Users/chuck/github/refinery-platform/heatmap-scatter-dash/context/app_runner.py", line 51, in main
    if args.files:
AttributeError: 'DefaultArgs' object has no attribute 'files'


ok