Maybe use MustSyncUntil?

tests/federation_room_join_partial_state_test.go

@@ -228,26 +228,21 @@ func TestPartialStateJoin(t *testing.T) {
 		// release the federation /state response
 		psjResult.FinishStateRequest()
 
-		t.Log("8. Have Alice eager-sync. She should see the remote room.")
-		response, eagerSyncToken = alice.MustSync(t, getEagerSyncReq())
-
-		roomRes := response.Get(syncJoinedRoomPath)
-		if !roomRes.Exists() {
-			t.Fatal("Sync should now include the joined room since resync is over")
+		t.Log("8. Have Alice eager-sync. She should see the remote room, including" +
+			"Charlie and Derek.")
+		remoteMembershipFor := func(localpart string) func(result gjson.Result) bool {
+			return func(result gjson.Result) bool {
+				return result.Get("type").Str == "m.room.member" && result.Get("state_key").Str == server.UserID(localpart)
+			}
 		}
 
-		// check that the state includes both charlie and derek.
-		matcher := match.JSONCheckOffAllowUnwanted("state.events",
-			[]interface{}{
-				"m.room.member|" + server.UserID("charlie"),
-				"m.room.member|" + server.UserID("derek"),
-			}, func(result gjson.Result) interface{} {
-				return strings.Join([]string{result.Map()["type"].Str, result.Map()["state_key"].Str}, "|")
-			}, nil,
+		_ = alice.MustSyncUntil(
+			t,
+			getEagerSyncReq(),
+			client.SyncJoinedTo(alice.UserID, serverRoom.RoomID),
+			client.SyncStateHas(serverRoom.RoomID, remoteMembershipFor("charlie")),
+			client.SyncStateHas(serverRoom.RoomID, remoteMembershipFor("derek")),
 		)
-		if err := matcher([]byte(roomRes.Raw)); err != nil {
-			t.Errorf("Did not find expected state events in /sync response: %s", err)
-		}
 	}
 
 	t.Run("EagerInitialSyncDuringPartialStateJoin", func(t *testing.T) {