Don't change PATH when activating the test virtualenv

[mppf]

Follow-up to PR #16560

The python interoperability tests rely on finding cython and numpy.
For these tests, we assume that these libraries are installed system-wide.
However, the Python test scripts have their own virtualenv which,
after PR #16560, includes a python3 command for the virtualenv that
is first in PATH. The test system activates a virtualenv to have access
to packages that support testing (notably PyYAML for import yaml and
filelock for import filelock).

This PR adjusts the Python code that activates the test virtualenv to avoid
modifying PATH. That way, the Python modules are still available in the
test Python program itself. But, it does not interfere with which python3
is invoked in a subprocess. Python scripts that are created as subprocesses
during testing need to import activate_chpl_test_venv in order to have
access to any dependencies from the virtualenv.

• full local testing
• spot-checked a performance test
• CHPL_TEST_ARKOUDA=true ./util/start_test test/studies/arkouda/ passes

Reviewed by @lydia-duncan - thanks!

[ben-albrecht]

This works, but seems fragile.

Rephrasing the problem, the main issue is that start_test has one set of python dependencies specified in test-venv and tests (such as Arkouda or Chapel-python-interop tests) have a different set of dependencies either specified in their own virtualenv or expected as system dependencies.

What we could do instead is deactivate the test virtualenv (test-venv) such that the test program runs in a clean environment, where it can source a new virtualenv or use system-installed dependencies.

Unfortunately, we are using test-venv/bin/activate_this.py because we activate test-venv from the system python rather than test-venv/bin/python, and virtualenv does not support a deactivate_this.py. So in order to deactivate, we'll need to implement that. There's an implementation provided in this SO post that may serve as a good starting point.

Once implemented, we could remove this fix and maybe change our test-venv-imports from this:

try:
    import activate_chpl_test_venv
    import filelock
except ImportError:
    pass

to this:

try:
    import activate_chpl_test_venv
    import filelock
    import dectivate_chpl_test_venv
except ImportError:
    pass

Note: we could do fancier things like a context manager or function to handle venv-dependent imports, but that feels like over-engineering at this point.