This updates the entire Djblets test suite to use

ExtensionTestCaseMixin for unit tests that need to work with

extensions, instead of ExtensionTestsMixin. The former is the public

interface we provide for testing capabilities, while the latter was an

in-house, hacky, outdated, and leaky mixin used by the bulk of (but not

all of) our tests.
The main reason for this change has to do with extension state leaks.

When moving to pytest, the Python 3.10 unit tests were failing due to

an extension not having the right state when a unit test was tearing

down.
The reason behind this is that extensions and extension managers weren't

being cleaned up properly. Many tests were creationg extension managers

that never got cleaned up. Since we use weak references for extension

managers, it's possible that the old test runner was allowing them to

clear in some of the cases, but the new one wasn't. Still, the logic

wasn't reliable, and this actually influenced the behavior of some of

our tests.
The modern mixin handles cleanup properly, and gives us some utilities

for managing extension scan state, simplifying some unit tests.
The old classes are still around, but will go through a deprecation

process. This is because we actually used to use the old classes for

extension unit tests, and before I rip it out, I want to make sure we

aren't still doing that anywhere.

All unit tests pass on all supported versions of Python and Django.