Verify batch coordinates in tests

[maxrjones]

For some tests the batch dimensions are checked but not the coordinates, as noted at https://github.com/pangeo-data/xbatcher/blob/34ca3f9b6acaf23b09ef140b69bc7c79d4f38feb/xbatcher/tests/test_generators.py#L167-L168 I think adding this type of verification would be helpful for preventing regressions while working on performance improvements (xref #37) and am interested in submitting a PR with this addition.