Mavlink: dont broadcast during unit-testing #27288

Job	Run time
build (check_format)	1m 9s
build (check_newlines)	1m 52s
build (tests)	5m 42s
build (tests_coverage)	7m 21s
build (px4_fmu-v2_default stack_check)	3m 24s
build (validate_module_configs)	52s
build (shellcheck_all)	1m 36s
build (NO_NINJA_BUILD=1 px4_fmu-v5_default)	4m 7s
build (NO_NINJA_BUILD=1 px4_sitl_default)	3m 21s
build (px4_sitl_allyes)	4m 29s
build (module_documentation)	1m 4s
	34m 57s

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