code: function firstSync(testClient) {
      // send a sync response including our test room.
      const syncData = {
          next_batch: "x",
          rooms: {
              join: { },
          },
      };
      syncData.rooms.join[roomId] = {
          state: {
              events: [
                  testUtils.mkMembership({
                      mship: "join",
                      user: aliUserId,
                  }),
                  testUtils.mkMembership({
                      mship: "join",
                      user: bobUserId,
                  }),
              ],
          },
          timeline: {
              events: [],
          },
      };
  
      testClient.httpBackend.when("GET", "/sync").respond(200, syncData);
      return testClient.flushSync();
  }
docs: Send an initial sync response to the client (which just includes the member
  list for our test room).
  
  @param {TestClient} testClient
  @returns {Promise} which resolves when the sync has been flushed.