Separate test framework extensions

If you use RSpec and include rantly/property, you drag in Test::Unit. If you use --fail-fast with RSpec, that argument goes off to Test::Unit's runner as well. Test::Unit doesn't understand that argument, so even if your tests all pass, the exit status indicates a failure. That makes your CI builds fail even though the tests all pass. This pull request lets Test::Unit people just use Test::Unit, and the RSpec people just use RSpec.