Use valid certificates in tests

Some of our tests use a roster with invalid certificates. This forces our code to be lenient, even though in production all certificates must be valid.

The tests should be updated with proper certificates, so that we can tighten the code dealing with rosters.

Locate the tests that use the roster with invalid certificates and inspect how those certificates are loaded and validated. Replace the invalid certificates with proper ones, then run the affected tests; done means the tests pass while representing production-valid rosters and no longer require lenient certificate handling.

Written by the indexing model from the issue text.