'--help-reactors with only supported reactors'

def test_onlySupportedReactors(self):

    def getReactorTypes():
        (yield twisted_reactors.default)
    options = app.ReactorSelectionMixin()
    options._getReactorTypes = getReactorTypes
    options.messageOutput = NativeStringIO()
    self.assertRaises(SystemExit, options.opt_help_reactors)
    message = options.messageOutput.getvalue()
    self.assertNotIn('reactors    not    available', message)
