CMake: Fix platform wait_ns greentea test Cmake

MBed CLI2 generates TARGET_ARM_FM macro as a string in mbed_config.cmake
under MBED_TARGET_DEFINITIONS list so fixed the if defined check of
TARGET_ARM_FM with finding TARGET_ARM_FM string in MBED_TARGET_DEFINITIONS
list to skip/include test

Author: rajkan01

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()