Fix a missing @requires_zarr in tests (#5936)
Fix a missing @requires_zarr in tests (#5936)

When zarr is not available, this test fails with `NameError: name 'zarr' is not defined`.
pydata/xarray,jhamman/xarray,markelg/xray,pydata/xarray,jhamman/xarray,markelg/xray,jhamman/xarray,pydata/xarray,markelg/xray