[simulation] enable the platform assert (#10952)

When the ot-rcp enters the assert state, the host crashes and exits.
But the ot-rcp still runs in the dead loop and becomes an orphan
process.

This commit enables the platform assert so that the ot-rcp can
automatically exit when entering the assert state.

tests/gtest/fake_platform.cpp

@@ -575,5 +575,5 @@ otError otPlatUdpLeaveMulticastGroup(otUdpSocket *, otNetifIdentifier, const otI
 {
     return OT_ERROR_NOT_IMPLEMENTED;
 }
-
+void otPlatAssertFail(const char *, int) {}
 } // extern "C"