Shim - mbed SDK uses us ticker by default

mbed SDK defines ``__MBED__`` macro (unique, not valid for yotta),
this should turn on us ticker by default, as that one is available
for mbed SDK targets.

Author: 0xc0170

frameworks\utest/utest/shim.h

@@ -59,7 +59,11 @@
 #       define UTEST_SHIM_SCHEDULER_USE_MINAR UTEST_MINAR_AVAILABLE
 #   endif
 #   ifndef UTEST_SHIM_SCHEDULER_USE_US_TICKER
-#       define UTEST_SHIM_SCHEDULER_USE_US_TICKER 0
+#       ifdef __MBED__
+#           define UTEST_SHIM_SCHEDULER_USE_US_TICKER 1
+#       else
+#           define UTEST_SHIM_SCHEDULER_USE_US_TICKER 0
+#       endif
 #   endif
 #endif  // YOTTA_CFG_UTEST_USE_CUSTOM_SCHEDULER