Coverage: Run lcov separate to fix paths

Without this the paths of the different compile units do not match up.

Makefile

@@ -368,6 +368,8 @@ tests_integration_coverage:
 	@$(MAKE) --no-print-directory px4_sitl_default sitl_gazebo
 	@$(MAKE) --no-print-directory px4_sitl_default mavsdk_tests
 	@"$(SRC_DIR)"/test/mavsdk_tests/mavsdk_test_runner.py --speed-factor 100
+	@mkdir -p coverage
+	@lcov --directory build/px4_sitl_default --base-directory build/px4_sitl_default --gcov-tool gcov --capture -o coverage/lcov.info
 
 tests_mission: rostest
 	@"$(SRC_DIR)"/test/rostest_px4_run.sh mavros_posix_tests_missions.test