On Python 3.10, we were getting other warnings mixed into the warnings the tests
were looking for.  This change lets us only look at the CoverageWarnings.

The auto-decorating metaclass was interfering with parameterized methods
on Python 2.7.  But we don't need it anymore anyway, since pytest will
let us hook to deal with the exception in a simpler way.

Another step toward removing unittest.TestCase.