FastModel: Skip low-power ticker time drifting test for models

FastModels are a software simulator for Arm platform/cores.
They mean to be used as a tool for testing or early prototype stage development.
Due to the nature of software simulator can't grantee the timing accuracy like HW.
So time drifting test is skipped for software models.

Author: Qinghao Shi

TESTS/mbed_drivers/lp_timeout/main.cpp

@@ -74,9 +74,10 @@ Case cases[] = {
     Case("1 s delay during deepsleep (attach_us)", test_deepsleep<AttachUSTester<LowPowerTimeout>, 1000000, LONG_DELTA_US>,
          greentea_failure_handler),
 #endif
-
+#if !defined(__ARM_FM)  //FastModels not support time drifting test
     Case("Timing drift (attach)", test_drift<AttachTester<LowPowerTimeout> >),
     Case("Timing drift (attach_us)", test_drift<AttachUSTester<LowPowerTimeout> >),
+#endif
 };
 
 utest::v1::status_t greentea_test_setup(const size_t number_of_cases)