Make sure extensions include tests where files are read purely on autogenerated classes

[rly]

TODO: encourage / ensure extensions include tests where files are read purely on autogenerated classes

[oruebel]

I think an alternative approach may be that we could require that an extension include a standard test-case to generate an example HDF5 file for the extension. We could include the outline for that test as part of the extension template so that the extension developer only needs to add the code to populate the NWBFile. Once we have a way to automatically generate an example HDF5 file for an extension, we should be able to add our own set of standard tests to: 1) run validation for the file, 2) read the file with autogenerated classes only, 3) read the file with the extension installed etc.