msc2836: fix flakey test caused by /send returning 404

This test uses a `Waiter` to wait for an incoming request from the HS.
If that request doesn't arrive in 1 second it fails the test. Previously,
we had UnexpectedRequestsAreErrors=false in this test to not treat requests
to /send as a failure. However, this would still 404 the /send endpoint. This
would cause Dendrite to begin backing off that server (starting with a 1s backoff)
which meant the Waiter would expire before /event_relationships would be sent.

The fix is simple: 200 OK /send transactions.

Author: kegsay

tests/msc2836_test.go

@@ -196,9 +196,8 @@ func TestFederatedEventRelationships(t *testing.T) {
 	srv := federation.NewServer(t, deployment,
 		federation.HandleKeyRequests(),
 		federation.HandleMakeSendJoinRequests(),
+		federation.HandleTransactionRequests(nil, nil),
 	)
-	// we expect to be pushed transactions but don't care about them
-	srv.UnexpectedRequestsAreErrors = false
 	cancel := srv.Listen()
 	defer cancel()