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

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/20241029031324.482800-1-lizhijian@fujitsu.com/
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>