Replace the janky have_problem matchers

This removes the kind of janky have_problem and only_have_problem
matchers with the new contain_(warning,error,fixed) matcher. This
greatly improves the readability of both the tests and their output on
failure.
Convert the plugin specs to the new matcher
Convert the rest of the specs to the new expect format
Update nested_classes_or_defines message

This annoyed me when I was fixing the specs, so I’m going to change it
now.
Update this spec too
Merge pull request #238 from rodjek/rejig_tests

Rejig tests