This fixes the test suite that @pypingou found broken.

It used to be that 'topic_prefix_re' was in the fedmsg default config,
but it has since been removed in development to make way for non-fedora
installations that will have a non-fedora topic_prefix (like debian).
Merge pull request #45 from fedora-infra/feature/fix-test-suite

This fixes the test suite that @pypingou found broken.