message: external_example: fix integration test build
diff: using namespace dronelink;
  -TEST(HelloWorld, SayHello)
  +TEST_F(SitlTest, ExternalExampleHello)
  {
  - sitl::start();
  -
  DroneLink dl;
  DroneLink::ConnectionResult ret = dl.add_udp_connection();
  @@ -23,6 +21,4 @@ TEST(HelloWorld, SayHello)
  // Apparently it can say hello.
  dl.device().example().say_hello();
  -
  - sitl::stop();
  }
  