Use PDU not *Event in HeaderedEvent (#3073)

Requires https://github.com/matrix-org/gomatrixserverlib/pull/376

This has numerous upsides:
 - Less type casting to `*Event` is required.
- Making Dendrite work with `PDU` interfaces means we can swap out Event
impls more easily.
 - Tests which represent weird event shapes are easier to write.

Part of a series of refactors on GMSL.

test/http.go

@@ -52,7 +52,7 @@ func NewRequest(t *testing.T, method, path string, opts ...HTTPRequestOpt) *http
 // ListenAndServe will listen on a random high-numbered port and attach the given router.
 // Returns the base URL to send requests to. Call `cancel` to shutdown the server, which will block until it has closed.
 func ListenAndServe(t *testing.T, router http.Handler, withTLS bool) (apiURL string, cancel func()) {
-	listener, err := net.Listen("tcp", ":0")
+	listener, err := net.Listen("tcp", "127.0.0.1:0")
 	if err != nil {
 		t.Fatalf("failed to listen: %s", err)
 	}

test/room.go

@@ -204,7 +204,7 @@ func (r *Room) CreateEvent(t *testing.T, creator *User, eventType string, conten
 	if err = gomatrixserverlib.Allowed(ev, &r.authEvents); err != nil {
 		t.Fatalf("CreateEvent[%s]: failed to verify event was allowed: %s", eventType, err)
 	}
-	headeredEvent := &rstypes.HeaderedEvent{Event: ev}
+	headeredEvent := &rstypes.HeaderedEvent{PDU: ev}
 	headeredEvent.Visibility = r.visibility
 	return headeredEvent
 }
@@ -215,7 +215,7 @@ func (r *Room) InsertEvent(t *testing.T, he *rstypes.HeaderedEvent) {
 	// Add the event to the list of auth/state events
 	r.events = append(r.events, he)
 	if he.StateKey() != nil {
-		err := r.authEvents.AddEvent(he.Event)
+		err := r.authEvents.AddEvent(he.PDU)
 		if err != nil {
 			t.Fatalf("InsertEvent: failed to add event to auth events: %s", err)
 		}