self-test fails if __pycache__ exists from prior run

[Bryce Harrington]

When running `./snap-wrappers/wrappers/git-ubuntu-self-test` multiple times on a git checkout (e.g. testing each commit in a MP), each subsequent run results in a series of errors like:

__________________________________ ERROR collecting gitubuntu/source_information.py __________________________________
import file mismatch:
imported module 'gitubuntu.source_information' has this __file__ attribute:
  /tmp/ci-20200509-ao1OztATiR/gitubuntu/source_information.py
which is not the same as the test file we want to collect:
  /tmp/ci-20200509-Otl7cY1ey8/gitubuntu/source_information.py
HINT: remove __pycache__ / .pyc files and/or use a unique basename for your test file modules

The hint provided is accurate: Simply `rm -rf ../usd-importer/gitubuntu/__pycache__` and re-run the test and it works. It would be more convenient if the test runner could do this for us. Guessing this might relate to how running this through the snap install behaves.