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)

[ilan-gold]

I've started looking into this and thought of a few more considerations:

1. Our gen_adata function uses types to filer keys in varm, layers etc. I'm not sure this is an ideal API but I am also fine with it. The issue is what those default keys are - we currently have two optional dependencies as defaults (awkward and xarray). I think both are valid (especially if DataFrame API for obs and var keys via runtime-checkable Protocol #2043 lands, we'll need a nice test-case for it). So, do we make a new extra for this functionality? Turn them off if they aren't installed and make it clear in the docs they are defaults otherwise? Remove them?
2. feat: support array-api #2071 contains a nice simplification I think, so probably best to wait for it to land before getting carried away with this.
3. assert_equal is in relatively good condition, I think and its guarantee to me seems to be clear - we aren't checking for types but rather numerical equality so don't expect that. We also probably don't want to make performance guarantees here since we denseify fairly aggressively.

assert_equal and gen_adata is probably all I would export - we cover data generation and assertion, you cover the test