tests: fix test suite runner not to require anymore the user to set PYTHONPATH before launching it.

[svalat]

When launching the test suite, there are som internal calls to pytools idefix module in sub commands.

Which leads to :

---------------------------------------------------- Captured stderr call -----------------------------------------------
Traceback (most recent call last):
  File "/....../idefix/test/Dust/DustyShock/python/testidefix.py", line 19, in <module>
    from pytools.vtk_io import readVTK
ModuleNotFoundError: No module named 'pytools.vtk_io'

It require set PYTHONPATH environment variable in front of ./tests.py to find the pytools module.

This patch, set the PYTHONPATH automatically at the start of ./tests.py so the user has nothing to do.