message: integration_tests: fix intermittent test fail
  Depending on timing, we sometimes tried to fulfil the promise twice
  which leads to an abort.
  This should fix the intermittent CI failure for the MavlinkPassthrough
  test:
  HIGHRES_IMU.temperature [1] (100)32 degrees C
  (mavlink_passthrough.cpp:61)
  terminate called after throwing an instance of 'std::system_error'
  what():  Resource deadlock avoided
diff: @@ -60,7 +60,7 @@ TEST_F(SitlTest, MavlinkPassthrough)
  LogInfo() << "HIGHRES_IMU.temperature [1] (" << counter << ")"
  << highres_imu.temperature << " degrees C";
  - if (++counter > 100) {
  + if (++counter == 100) {
  EXPECT_FALSE(stopped);
  if (!stopped) {
  stopped = true;
  