Move tests out of the package

[flying-sheep]

We currently have unit tests inside of the anndata package. We should separate them out following the recommended structure from Pytest (that we use in the scverse template).

Not having them separated also creates an issue with the src layout and --pyargs:

1. We excise tests from the package during packaging using a exclude config

2. this means that running tests on an editable install vs a regular install works differently:

   • In an editable install, pytest --pyargs anndata will run the unit tests inside of anndata/tests. ✅
   • In an editable install, pytest --pyargs anndata ./anndata/tests will run the unit tests twice
   • In a regular install, pytest --pyargs anndata will only run doctest since there are no unit tests in anndata/tests
   • In a regular install, pytest --pyargs anndata ./anndata/tests will run doctests and unit tests ✅