'Test that --help-auth will only list checkers that purport to
supply at least one of the credential interfaces our
application can use.'

def test_helpAuthLimitsOutput(self):

    options = OptionsForUsernamePassword()
    for factory in options._checkerFactoriesForOptHelpAuth():
        invalid = True
        for interface in factory.credentialInterfaces:
            if options.supportsInterface(interface):
                invalid = False
        if invalid:
            raise strcred.UnsupportedInterfaces()
