BUG: testing.run_module_suite() didn't pass on KnownFailure.

This results in errors whenever you run tests using KnownFailure
like so: ``python test_mymodule.py`` (where run_module_suite is
used in the file, like numpy and scipy do).
Merge pull request #4211 from rgommers/fix-knownfail

BUG: testing.run_module_suite() didn't pass on KnownFailure plugin.