Don't overwrite PYTHONPATH if running unit tests
Don't overwrite PYTHONPATH if running unit tests

This behavior was observed when running the Python tests from Waf
(python ./waf run_python_tests)
The script creates a local Python package and sets PYTHONPATH to run the
tests ignoring existing entries
This commit proposes appending our local package to the existing
PYTHONPATH to prevent ignoring existing entries

MTG/essentia,MTG/essentia,MTG/essentia,MTG/essentia,MTG/essentia