TST: include mock classes into coverage run

It seems like that some of the mock classes we use in tests are extremely detailed, yet the nuances are not used in the tests creating the impression we cover more corner cases than we actually do.

With a bit of file restructuring, I expect that it would be possible to include those classes into the coverage run, (or have a second coverage run that only runs for the test mocks?, details TBD)

https://github.com/astropy/pyvo/pull/432#issuecomment-1458533896

Review the test mock classes and the repository's current coverage configuration, then trace which mock behavior is currently excluded. Determine whether restructuring or a separate mock-focused coverage run is appropriate; done means the detailed mock behavior is exercised and the coverage results no longer imply untested corner cases are covered.

Written by the indexing model from the issue text.