Merge pull request #15066 from ARMmbed/fix_platform_wait_ns_test_cmake

CMake: Fix platform wait_ns greentea test Cmake

Author: Patater

platform/tests/TESTS/mbed_platform/wait_ns/CMakeLists.txt

@@ -3,7 +3,7 @@
 
 include(mbed_greentea)
 
-if(DEFINED TARGET_ARM_FM)
+if("TARGET_ARM_FM" IN_LIST MBED_TARGET_DEFINITIONS)
     set(TEST_SKIPPED "Wait ns test not supported.")
 endif()