Fix PYTHONPATH for tests when using a venv

[JCGoran]

When building NMODL with the default flags, it’s possible to get errors like:

/test/unit/visitor/sympy_solver.cpp:2233: FAILED:
  REQUIRE_NOTHROW( result = run_sympy_solver_visitor( nmodl_text, false, false, AstNodeType::DERIVATIVE_BLOCK, true) )
due to unexpected exception with message:
  ModuleNotFoundError: No module named 'find_libpython'

At:
    /build/lib/nmodl/__init__.py(9): <module>
    <frozen importlib._bootstrap>(228): _call_with_frames_removed
    <frozen importlib._bootstrap_external>(850): exec_module
    <frozen importlib._bootstrap>(695): _load_unlocked
    <frozen importlib._bootstrap>(986): _find_and_load_unlocked
    <frozen importlib._bootstrap>(1007): _find_and_load
    <frozen importlib._bootstrap>(228): _call_with_frames_removed
    <frozen importlib._bootstrap>(972): _find_and_load_unlocked
    <frozen importlib._bootstrap>(1007): _find_and_load
    <string>(3): <module>

when running ctest, even though you may have installed all of the requirements via pip in your virtualenv beforehand (and pip show find_libpython finds it). The reason is this line here:

nmodl/test/unit/CMakeLists.txt

Line 119 in 347f786

set(testvisitor_env "PYTHONPATH=${PROJECT_BINARY_DIR}/lib:$ENV{PYTHONPATH}")

This PR fixes this by using the workaround from this SO answer to prepend PYTHONPATH with the path to where packages are installed in the venv (should work outside of the venv as well).

Note that, as the answer mentions, the Python that comes with Debian is a bit wonky, and I'm not sure if this patch fixes it also on that platform (I tested it on a Debian Bullseye container, aka currently "oldstable", and it seems to work), but at least it shouldn't introduce any regressions.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.58%. Comparing base (347f786) to head (c9ba53e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1208   +/-   ##
=======================================
  Coverage   86.58%   86.58%           
=======================================
  Files         175      175           
  Lines       13006    13006           
=======================================
  Hits        11261    11261           
  Misses       1745     1745           

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[bbpbuildbot]

Logfiles from GitLab pipeline #199123 (:white_check_mark:) have been uploaded here!

Status and direct links:

• ✅ spack_setup
• ✅ build:intel
• ✅ build:nvhpc
• ✅ test:intel
• ✅ test:nvhpc

[1uc]

This has been confirmed to be a real bug, and not just a problem while testing/running CI.