Warn about unused mocks

[klajo]

Warn (or fail) when unused mocks are detected. This shall be done in a way that causes no false warnings. It is for example not enough to plug in a check at the end of a fixture (multi-testcase fixture), since there can be multiple fixtures in a test module and those could use a common setup/0 function (which can be convenient to simplify the test code).

I have some experiments in progress.