sys.kern.pdeathsig.signal_delivered_ptrace: fix debugger detach

ptrace(PT_DETACH) requires stopped debuggee, otherwise it fails.  When
the call fails, the C process is left as debuggee of the process D,
and might be killed too early if process D exits occurs fast enough.

Since pipes are not closed in the forked children, this resulted in
the test hanging, since no write occured from C to wake A.

PR:	237657
Reported and tested by:	lwhsu
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

tests/sys/kern/pdeathsig.c

@@ -305,7 +305,9 @@ ATF_TC_BODY(signal_delivered_ptrace, tc)
 				    WSTOPSIG(status));
 			assert(rc == 0);
 
-			ptrace(PT_DETACH, c_pid, 0, 0);
+			waitpid(c_pid, &status, 0);
+			if (!WIFEXITED(status))
+				ptrace(PT_DETACH, c_pid, 0, 0);
 
 			_exit(0);
 		}