Cleanups and Speedup for python unittests

[dcbaker]

I initially set out to just do some long overdue maintenance of our unit tests, along way I cleaned up a number of issues that have been bugging me for a while:

• use mock instead of try/finally blocks
• use os.enivon.get so that some things don't have to be set to run
• move a lot of constant setup to setUpClass instead of setUp, which means the fixtures get run less
• use more of our helpers instead of open coding
• make more use of UnitTest.addCleanup
• actually clean up more often, and move the cleanup code closer to the producer.
• Allow our cleanup of Meson calls to happen even if the call to Meson itself fails
• cleanup some complicated logic
• use better/faster types
• use properties for rarely used dynamic attributes
• Use tmpdir instead of source dir when we can reasonably assume that there will not be Anti-Virus, and allow Windows users to force the use of the tmpdir

For me the end result is a reduction in a full unit test run time by ~15%, and there are no stray files left in the source tree compared to the main branch.

I wont be surprised if there are a few regressions on Windows to clean up.

[dcbaker]

I've also reworded a few of the commit messages to fix typos, but I'll wait to re-push so I don't flood the CI

[dcbaker]

Squashed and re-ordered a bit to address comments from IRC/Matrix. Attempting to fix the one test that is failing on Windows

[eli-schwartz]

Partial review and partial merge of commits 85e92331c...4b76aabe3

[jpakkane]

Other than the one change LGTM.