Admin tests fail locally but pass in CI

[eloquence]

Steps to reproduce

1. Run make -C admin test

Expected behavior

All tests pass

Actual behavior

================================================================================== short test summary info ===================================================================================
FAILED tests/test_securedrop-admin-setup.py::TestSecureDropAdmin::test_install_pip_dependencies_up_to_date - FileNotFoundError: [Errno 2] No such file or directory: '/sd-root/admin/.venv3/bin/python3'
FAILED tests/test_securedrop-admin-setup.py::TestSecureDropAdmin::test_install_pip_dependencies_upgraded - FileNotFoundError: [Errno 2] No such file or directory: '/sd-root/admin/.venv3/bin/python3'
FAILED tests/test_securedrop-admin-setup.py::TestSecureDropAdmin::test_install_pip_dependencies_fail - FileNotFoundError: [Errno 2] No such file or directory: '/sd-root/admin/.venv3/bin/python3'
==================================================================== 3 failed, 76 passed, 1 warning in 161.27s (0:02:41) =====================================================================
ERROR: InvocationError for command /usr/bin/env .tox/py3/bin/coverage run --source=securedrop_admin,bootstrap .tox/py3/bin/pytest -v tests (exited with code 1)

Analysis

AFAICT we're bootstrapping the venv into the dev container and use that in CI, but in local development, we're volume-mounting the repository root and using that as the WORKDIR. I assume the tests will be happy if one creates a .venv3, but that shouldn't be necessary and leads to confusing mismatches between CI vs. local dev results.

[eloquence]

We could just error out in the Makefile if the venv doesn't exist. Because it's able to successfully run most of the tests in the dev container, just not the pip-related ones, the current behavior is a bit confusing.

Alternatively, since we're already baking a venv into the Docker image using bootstrap.py, we could copy that one into the volume mount during local runs if the venv is missing.

[legoktm]

Note that this setup is very bad (c.f. #6897) so making bigger/radical changes is totally fine IMO.