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

@@ -368,18 +368,19 @@ list(APPEND CMAKE_MODULE_PATH ${PX4_SOURCE_DIR}/cmake/gtest/)
 include(px4_add_gtest)
 if(BUILD_TESTING)
 	include(gtest)
+
+	add_custom_target(test_results
+			COMMAND GTEST_COLOR=1 ${CMAKE_CTEST_COMMAND} --output-on-failure -T Test -R ${TESTFILTER} USES_TERMINAL
+			DEPENDS
+				px4
+				examples__dyn_hello
+				test_mixer_multirotor
+			USES_TERMINAL
+			COMMENT "Running tests"
+			WORKING_DIRECTORY ${PX4_BINARY_DIR})
+	set_target_properties(test_results PROPERTIES EXCLUDE_FROM_ALL TRUE)
 endif()
 
-add_custom_target(test_results
-		COMMAND GTEST_COLOR=1 ${CMAKE_CTEST_COMMAND} --output-on-failure -T Test -R ${TESTFILTER} USES_TERMINAL
-		DEPENDS
-			px4
-			examples__dyn_hello
-			test_mixer_multirotor
-		USES_TERMINAL
-		COMMENT "Running tests"
-		WORKING_DIRECTORY ${PX4_BINARY_DIR})
-set_target_properties(test_results PROPERTIES EXCLUDE_FROM_ALL TRUE)
 
 #=============================================================================
 # subdirectories