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

cmd/dendrite-monolith-server/main.go

@@ -135,7 +135,7 @@ func main() {
 	monolith.AddAllPublicRoutes(base.PublicAPIMux)
 
 	httputil.SetupHTTPAPI(
-		http.DefaultServeMux,
+		base.BaseMux,
 		base.PublicAPIMux,
 		base.InternalAPIMux,
 		cfg,
@@ -147,6 +147,7 @@ func main() {
 		serv := http.Server{
 			Addr:         *httpBindAddr,
 			WriteTimeout: setup.HTTPServerTimeout,
+			Handler:      base.BaseMux,
 		}
 
 		logrus.Info("Listening on ", serv.Addr)
@@ -158,6 +159,7 @@ func main() {
 			serv := http.Server{
 				Addr:         *httpsBindAddr,
 				WriteTimeout: setup.HTTPServerTimeout,
+				Handler:      base.BaseMux,
 			}
 
 			logrus.Info("Listening on ", serv.Addr)