ostest: fix assert when open CONFIG_SIG_DEFAULT

SIGKILL was captured and call nxsig_abnormal_termination,
which kills the children when SIGCHLD was set. Solution:
change SIGKILL to SIGUSR1.

Signed-off-by: qiaowei <qiaowei@xiaomi.com>
Change-Id: I4a4b06220a28fcf9d50debfd8a3b789fdbdf1976

testing/ostest/mqueue.c

@@ -359,7 +359,7 @@ void mqueue_test(void)
   /* Wake up the receiver thread with a signal */
 
   printf("mqueue_test: Killing receiver\n");
-  pthread_kill(receiver, 9);
+  pthread_kill(receiver, SIGUSR1);
 
   /* Wait a bit to see if the thread exits on its own */