Follow-up from "Updates for python3 "native""

The following discussion from !1418 (merged) should be addressed:

• @graemes started a discussion: (+1 comment)

  I did have one uncertainty about the current PR, which is that in the testGaudiToolbox.cmake file the PYTHONPATH is set to the Gaudi sources directory, because the dummy project Python in the build doesn't have the right Python module files:

  ➜  Gaudi ls dummyProjectBinaryDir/python/dummyProject
  __init__.py  __pycache__

  Is this expected? Or should the dummyModule.py and dummy.py files have been copied to the build area.

  It wasn't very clear to me.