🧪 TESTS: Entirely remove AiidaTestCase

[chrisjsewell]

bye bye 👋

[ltalirz]

Thanks a lot @chrisjsewell !

I don't have the time to go through all of the 85 modified files here; if there's something particular I should have a look at, let me know.

[chrisjsewell]

Heya, erm not particularly; in #5330 I already ensured consistency between the AiidaTestCase and the aiida_profile_clean test fixture, then this PR just follows the conversion process I outlined in #4782

So, personally, I'm happy that everything here is fine, and obviously all the tests are still passing, which is the main thing.

[chrisjsewell]

Ok well @sphuber has swooped in and approved lol, @ltalirz are you happy for me to merge, or did you still want to look at anything?

[ltalirz]

All good, go ahead