Add official testing.anndata API

[flying-sheep]

Please describe your wishes and possible alternatives to achieve the desired result.

AnnData.tests.helpers is an undocumented mess

We should clearly distinguish

• between non-pytest helpers (testing.anndata) and a pytest plugin (testing.anndata.pytest). Only the second would import pytest and e.g. define the GPU marker (see Move test helpers rapids_singlecell#267)
• between public and private helpers

And we should

• document how exactly things like assert_equal work and what they support. (As well as that they’re singledispatch and therefore extensible)