Add PyNWB tests

[oruebel]

https://github.com/NeurodataWithoutBorders/pynwb/pull/1018 we updates the PyNWB test harness to add ZarrIO to the rountrip tests, which in turn runs all HDF5 roundtrip tests that are defined in PyNWB also for Zarr. This requires changing the test harness in PyNWB, instead it would be useful to be able to “inject” new I/O backends in the PyNWB test harness so that we can specify those tests here, rather than implementing this in PyNWB and making PyNWB dependent on hdmf-zarr.

Note: The following changes from https://github.com/NeurodataWithoutBorders/pynwb/pull/1018 have already been ported:

• docs/notebooks/zarr_file_conversion_test.ipynb from the the PyNWB PR has been ported to docs/gallery/plot_convert_nwb.py here
• The changes in src/pynwb/__init__.py from the PyNWB PR have been added in src/hdmf_zarr/nwb.py here However, the changes to the tests have not been ported (at least not fully) and the test harness of PyNWB has undergone some refactoring in the meantime as well so we'll need to check how to best implement these tests

[oruebel]

This is not required for release but would be nice to setup as part of the CI to ensure broader test coverage. In particular, this will help keep Zarr tests up-to-date as all integration tests implemented for HDF5 will automatically become available to Zarr as well.