Verify that requests objects from clients are not valid client assertions

As per discussion from https://github.com/oauthstuff/draft-oauth-par/issues/41#issuecomment-617856097 it would be worthwhile to have a test condition in one of the existing FAPI-RW RP test modules that verifies that request objects clients create are not valid client assertions, i.e. that they do not include 'sub' claim that contains the client_id.