Expand testing section in developer docs

[ianmkenney]

There are a few points that would be useful to make for new developers regarding the testing protocol.

1. The inclusion of the pytest-xdist package in test.yml gives the impression that tests can be run in parallel, which appears only be true for the unit tests, not the integration tests. This leads to a wall of errors that could lead to:
2. interruption of the tests doesn't cleanly remove containers/volumes/networks. Rerunning the tests creates duplicate docker networks that will cause many future tests to fail. There needs to be a few sentences on resolving this duplication problem.