Don't import from tests

[d-v-b]

In main, DummyStorageTransformer is defined in the test suite and imported by zarr/storage/v3_storage_transformers. Because the distributed version of zarr should not be importing code from the tests, I moved the definition of DummyStorageTransformer to zarr/storage/v3_storage_transformers, and now the tests that use this class can import it like any other zarr code.

In the test suite, I also changed imports of the form from zarr.test.module.. import bla to from .module import bla.

Both of these changes are necessary for moving to a src/zarr layout.

xref: #1592

[d-v-b]

Does anyone have a problem with merging this? It will unblock efforts to modernize the packaging of zarr-python over in #1592

[d-v-b]

if nobody objects, I'm going to merge this in under 12 hours.

[d-v-b]

@jhamman has offered to review, so I will hold off on merging unilaterally

[jhamman]

Thanks @d-v-b! Just one question below.

[d-v-b]

This is going in with temporary release notes that distinguish between v2 and v3 contributions. When we merge v3 into main, we will revert this alteration to the release notes.