Streamline addon test suites

[jasonvarga]

As shown in #1159 and probably some other issues, testing addons needs some love.

I think it would be good to have our own abstract TestCase for developers to extend that does extra necessary bootstrapping, rather than them needing to extend from Testbench's and do a bunch of wiring.

There's also the ExceptionHandler issue, and Statamic config loading, both of which could probably be done as part of this abstracted class.

There's a bunch of useful fake classes used in our own tests that could be useful to addon devs. Stuff should probably be moved out from our tests directory and into a real Statamic\Testing namespace.

[github-actions]

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