It didn't work for me, because nose would crawl inside manual_tests, try
to run test_fstest.py, and fail (because that test is manual and
requires extra setup, like putting fstest.py on PYTHONPATH).

I also don't think that we want to exclude test layers from coverage
measurements (nose doesn't support them).

This change makes 'tox -e coverage' use zope.testrunner like it does for
regular tests.