Fix rooms v3 url paths for good - with tests (#1130)

* Fix rooms v3 url paths for good - with tests

- Add a test rig around `federationapi` to test routing.
- Use `JSONVerifier` over `KeyRing` so we can stub things out more easily.
- Add `test.NopJSONVerifier` which verifies nothing.
- Add `base.BaseMux` which is the original `mux.Router` used to spawn public/internal routers.
- Listen on `base.BaseMux` and not the default serve mux as it cleans paths which we don't want.
- Factor out `ListenAndServe` to `test.ListenAndServe` and add flag for listening on TLS.

* Fix comments

* Linting

federationapi/routing/leave.go

@@ -113,7 +113,7 @@ func SendLeave(
 	request *gomatrixserverlib.FederationRequest,
 	cfg *config.Dendrite,
 	rsAPI api.RoomserverInternalAPI,
-	keys gomatrixserverlib.KeyRing,
+	keys gomatrixserverlib.JSONVerifier,
 	roomID, eventID string,
 ) util.JSONResponse {
 	verReq := api.QueryRoomVersionForRoomRequest{RoomID: roomID}