TST: include mock classes into coverage run

[bsipocz]

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)

#432 (comment)