This first patch is committed!  If you have any question about the changes I made to your patch, feel free to ask them.  The most important change I did was running setUp and tearDown before and after each test method, as real unittest would do.

If you want to you write more tests, know that there are some things I want to remove or merge.  TestDistribution and create_distribution will probably be removed when I manage to work on my config module rewrite.

On the contrary, LoggingCatcher and EnvironRestorer won’t go away, so someone interested can work on testing them (LoggingCatcher: medium to hard, EnvironRestorer: piece of cake).

Finally, I’m not sure it’s useful to test two specific functions.  copy_xxmodule_c is tested by running the tests in different ways (via python -m test, python Lib/packaging/tests/test_thing.py, ../../../python test_thing.py); that’s how I made sure it worked in different situations, and tests wouldn’t have helped.  For fixup_build_ext, we know it works because we have buildbots compiling Python with various options (shared on Unix, debug on Windows) and our tests pass, so fixup_build_ext is doing its job.