Testing improvements

[maxrjones]

What is your issue?

As mentioned in https://github.com/pangeo-data/xbatcher/pull/71#pullrequestreview-1073348007, the xbatcher test suite needs improvements. This issue will track necessary improvements to the tests (additions are welcome):

• [x] Test data loaders on both datasets and dataarrays (done for pytorch at #85, still need to do for keras/tensorflow)
• [ ] Make data loader tests more granular
• [ ] Test batch generation on datasets with multi-dimensional coordinates
• [ ] Test batch generation on datasets with non-dimensional coordinates
• [ ] Test batch generation on datasets with other index types
• [x] Use xarray testing methods on batches for more informative diffs
• [x] Add a test for batch_dims (https://github.com/xarray-contrib/xbatcher/issues/121)