Bug 21503: Tests cleanup and highlight a missing case

Bug 21503: Tests cleanup and highlight a missing case

This patch does the following changes:
- Use build_object to create the MSS
- Store the generate AVs. That was not the case before and so we were
not testing the right things
- Split the tests into 2 parts: staff and OPAC, for readability
- Update the tests' descriptions
- Highlight a problem: If descriptions are missing for both OPAC and
staff, undef is returned. Is that what we expect? It seems that it is
not what were expecting the tests.

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>