fix(tests): bash subprocess tests source ~/.bashrc under SSH and fail #2306

Describe the bug
Tests that run subprocess.run(['bash', '-e', '-c', …]) inherit SSH_CLIENT. Non-interactive bash sources ~/.bashrc when SSH_CLIENT is set or stdin is a socket, and SHLVL is below 2. Its output and PATH then leak into the step under test.

Reproduction
From an SSH session, SHLVL=0 python -m pytest tests/project/test_apt_packages_1967.py::TestTheFormerShellAgrees::test_seed tests/project/test_project_status.py::TestTheStepRunsTheFetchedScript::test_both_files_are_fetched_and_the_import_finds_the_helper gives SUBFAILED(job='engine-tests', restored=True) and pypcap-parity: (0, '~/.bashrc: line 14: autoload: command not found…') != (0, 'Seeded 3 .deb files\n', …). In the second, the bashrc's PATH puts the real gh before the stub: gh: Bad credentials (HTTP 401).

Expected behavior
Each step's shell runs as GitHub Actions runs it, bash --noprofile --norc. Measured: with --norc, SSH_CLIENT set and a socket stdin, no bashrc runs.

Additional context
Adding stdin=subprocess.DEVNULL is not enough: pytest already gives fd 0 /dev/null, and the failure above came from SSH_CLIENT. Fix every ['bash', …] call in tests/project/, ideally through one helper in tests/_support. CI does not set SSH_CLIENT, so this is local only.