Fix bugs in handling clientRedirectUrl, and improve OIDC tests (#9127, #9128)

* Factor out a common TestHtmlParser

Looks like I'm doing this in a few different places.

* Improve OIDC login test

Complete the OIDC login flow, rather than giving up halfway through.

* Ensure that OIDC login works with multiple OIDC providers

* Fix bugs in handling clientRedirectUrl

 - don't drop duplicate query-params, or params with no value
 - allow utf-8 in query-params

synapse/handlers/oidc_handler.py

@@ -85,7 +85,7 @@ class OidcHandler:
         self._token_generator = OidcSessionTokenGenerator(hs)
         self._providers = {
             p.idp_id: OidcProvider(hs, self._token_generator, p) for p in provider_confs
-        }
+        }  # type: Dict[str, OidcProvider]
 
     async def load_metadata(self) -> None:
         """Validate the config and load the metadata from the remote endpoint.