code: function getSyncResponse(roomMembers) {
      const roomResponse = {
          state: {
              events: [
                  testUtils.mkEvent({
                      type: 'm.room.encryption',
                      skey: '',
                      content: {
                          algorithm: 'm.megolm.v1.aes-sha2',
                      },
                  }),
              ],
          },
      };
  
      for (let i = 0; i < roomMembers.length; i++) {
          roomResponse.state.events.push(
              testUtils.mkMembership({
                  mship: 'join',
                  sender: roomMembers[i],
              }),
          );
      }
  
      const syncResponse = {
          next_batch: 1,
          rooms: {
              join: {},
          },
      };
      syncResponse.rooms.join[ROOM_ID] = roomResponse;
      return syncResponse;
  }
docs: get a /sync response which contains a single room (ROOM_ID),
  with the members given
  
  @param {string[]} roomMembers
  
  @return {object} event