cmake: fix warning

When not building for testing, test_mixer_multirotor is not available
and causes a waraning with cmake 3.17:

  This project specifies custom command DEPENDS on files in the build tree
  that are not specified as the OUTPUT or BYPRODUCTS of any
  add_custom_command or add_custom_target:

   test_mixer_multirotor

CMakeLists.txt

@@ -368,9 +368,8 @@ list(APPEND CMAKE_MODULE_PATH ${PX4_SOURCE_DIR}/cmake/gtest/)
 include(px4_add_gtest)
 if(BUILD_TESTING)
 	include(gtest)
-endif()
 
-add_custom_target(test_results
+	add_custom_target(test_results
 			COMMAND GTEST_COLOR=1 ${CMAKE_CTEST_COMMAND} --output-on-failure -T Test -R ${TESTFILTER} USES_TERMINAL
 			DEPENDS
 				px4
@@ -379,7 +378,9 @@ add_custom_target(test_results
 			USES_TERMINAL
 			COMMENT "Running tests"
 			WORKING_DIRECTORY ${PX4_BINARY_DIR})
-set_target_properties(test_results PROPERTIES EXCLUDE_FROM_ALL TRUE)
+	set_target_properties(test_results PROPERTIES EXCLUDE_FROM_ALL TRUE)
+endif()
+
 
 #=============================================================================
 # subdirectories