selftests/watchdog-test: Fix system accidentally reset after watchdog-test

When running watchdog-test with 'make run_tests', the watchdog-test will
be terminated by a timeout signal(SIGTERM) due to the test timemout.

And then, a system reboot would happen due to watchdog not stop. see
the dmesg as below:
```
[ 1367.185172] watchdog: watchdog0: watchdog did not stop!
```

Fix it by registering more signals(including SIGTERM) in watchdog-test,
where its signal handler will stop the watchdog.

After that
# timeout 1 ./watchdog-test
Watchdog Ticking Away!
.
Stopping watchdog ticks...

Link: https://lore.kernel.org/all/[e-mail]/
Signed-off-by: Li Zhijian <[e-mail]>
Reviewed-by: Shuah Khan <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/watchdog/watchdog-test.c

···
 
 	printf("Watchdog Ticking Away!\n");
 
+	/*
+	 * Register the signals
+	 */
 	signal(SIGINT, term);
+	signal(SIGTERM, term);
+	signal(SIGKILL, term);
+	signal(SIGQUIT, term);
 
 	while (1) {
 		keep_alive();