message: ftp: fix integration tests
  We need to discover the system first before we can access it.
diff: @@ -168,11 +168,14 @@ TEST(FtpTest, ListDirectory)
  ConnectionResult ret = mavsdk.add_udp_connection();
  ASSERT_EQ(ret, ConnectionResult::Success);
  - auto system = mavsdk.systems().at(0);
  - auto ftp = std::make_shared<Ftp>(system);
  // Wait for system to connect via heartbeat.
  std::this_thread::sleep_for(std::chrono::seconds(2));
  +
  + ASSERT_EQ(mavsdk.systems().size(), 1);
  + auto system = mavsdk.systems().at(0);
  + auto ftp = std::make_shared<Ftp>(system);
  +
  // Reset server in case there are stale open sessions
  reset_server(ftp);
  @@ -185,11 +188,14 @@ TEST(FtpTest, DownloadFile)
  ConnectionResult ret = mavsdk.add_udp_connection();
  ASSERT_EQ(ret, ConnectionResult::Success);
  - auto system = mavsdk.systems().at(0);
  - auto ftp = std::make_shared<Ftp>(system);
  // Wait for system to connect via heartbeat.
  std::this_thread::sleep_for(std::chrono::seconds(2));
  +
  + ASSERT_EQ(mavsdk.systems().size(), 1);
  + auto system = mavsdk.systems().at(0);
  + auto ftp = std::make_shared<Ftp>(system);
  +
  // Reset server in case there are stale open sessions
  reset_server(ftp);
  @@ -217,11 +223,14 @@ TEST(FtpTest, UploadFiles)
  ConnectionResult ret = mavsdk.add_udp_connection();
  ASSERT_EQ(ret, ConnectionResult::Success);
  - auto system = mavsdk.systems().at(0);
  - auto ftp = std::make_shared<Ftp>(system);
  // Wait for system to connect via heartbeat.
  std::this_thread::sleep_for(std::chrono::seconds(2));
  +
  + ASSERT_EQ(mavsdk.systems().size(), 1);
  + auto system = mavsdk.systems().at(0);
  + auto ftp = std::make_shared<Ftp>(system);
  +
  // Reset server in case there are stale open sessions
  reset_server(ftp);
  