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/dendritejs/jsServer.go

@@ -28,7 +28,7 @@ import (
 // JSServer exposes an HTTP-like server interface which allows JS to 'send' requests to it.
 type JSServer struct {
 	// The router which will service requests
-	Mux *http.ServeMux
+	Mux http.Handler
 }
 
 // OnRequestFromJS is the function that JS will invoke when there is a new request.