remove xfail on `test_to_zarr_zip_store` in `test_backends_datatree`

[dcherian]

What is your issue?

Adding it in #10485 to avoid some Zarr breakage.

[TomNicholas]

These xfails can hopefully be removed once we have an upstream solution to round-tripping to ZipStore - see zarr-developers/zarr-python#3194