Tests should expose breaking changes to source data structure

[rjsparks]

There are several places where the bibxml code is having to adjust to changes in the relaton source data, and tests didn't show that things had become disconnected.

This needs to be addressed in addition to whatever actions come from the conversations around versioning libraries to help avoid encountering breaking changes by surprise in the future.

References:
#282 (review)
#283

[strogonoff]

As discussed with Kesara today, I don’t think the correctness of this Django project codebase should depend on the correctness of data sources and don’t want this project’s tests to also indirectly test Ruby gems (as well as network connectivity to Github repositories, etc.). We’ll look into making separate integration test scripts, like test_paths.py which was created for this purpose, run on GHA CI periodically.