test-watchdog: mark as unsafe

If something goes wrong with this test it may result in an unsafe
system restart. Let's avoid running it automatically.

See https://github.com/systemd/systemd/issues/22001.

src/test/meson.build

@@ -552,7 +552,8 @@ tests += [
          [],
          core_includes, '', 'manual'],
 
-        [['src/test/test-watchdog.c']],
+        [['src/test/test-watchdog.c'],
+         [], [], [], '', 'unsafe'],
 
         [['src/test/test-sched-prio.c'],
          [libcore,