Stop using the mock library for testing

[exarkun]

Some discussion about motivations for this

• ​https://nedbatchelder.com/blog/201206/tldw_stop_mocking_start_testing.html to start
• ​​https://pythonspeed.com/articles/verified-fakes/
• ​​https://martinfowler.com/articles/mocksArentStubs.html

Instead:

• Where possible, we will completely remove whatever role the mock is serving from the test and focus on more well-defined, isolated units of functionality.
• Failing that, where possible, we will use the implementation that is used under real usage of whatever the relevant interface is.
• Failing that, we will use verified fakes of whatever the relevant interface is.

This is a tracking ticket for various other tickets which make progress towards this goal. There are too many uses of mock to address this all at once. Fortunately each use should be essentially independent from the rest so the task is easily broken up.

Individual implementation tickets