[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/unit/test_platform.cpp

@@ -967,4 +967,5 @@ OT_TOOL_WEAK void otPlatDnssdStopIp4AddressResolver(otInstance *aInstance, const
 OT_TOOL_WEAK otError otPlatLogCrashDump(void) { return OT_ERROR_NONE; }
 #endif
 
+OT_TOOL_WEAK void otPlatAssertFail(const char *, int) {}
 } // extern "C"