ExtensionTestCaseMixin had a basic attempt at cleaning up state, but

it never actually unregistered extension classes, and any additional

extensions registered during a unit test would leak into other test

runs.
Now, it carefully goes through all new extensions that were registered,

and new extension managers, and tries to clear them out. This logic

lives in the new reset_extensions() method, which is called on

tearDown() but can also be called manually if a test requires it.
A new setup_extension() method (ported over from the legacy

ExtensionTestsMixin and enhanced) can be used by unit tests to

register and construct other extensions. setUp() now calls this when

the class-level extension definition attributes are provided.
New class-level attributes have also been added to control whether an

extension should be enabled by default, and for the default extension

manager class.
Previously, get_extension_manager() would require an existing

extension manager to be created, and would use it, but this led to

issues when extension managers leaked, or for projects that didn't

establish one for the test run. Now, it'll use an existing one if found

(unless allow_existing_extension_manager is turned off), or

instantiate the default extension manager class for the test run.
To make extensions available for ExtensionManager.load(), they can

call with self.scanned_extensions([...]) with a list of extension

classes that have been already set up using setup_extension().
This is a pretty comprehensive set of improvements, and will allow our

internal tests to switch over from the old ExtensionTestsMixin. That

will be done in a future change to fix the rest of the extension state

leaks.
As a new feature, this will require a .y version bump (to 2.3).

All unit tests pass under the old nose test runner.
All unit tests pass under the new pytest test runner, in combination

with test suite updates to use this class.