Fix a race on some platforms in mysql-test-run.pl, where it would sometimes

errorneously abort reporting failure to kill child processes, where in
reality the problem was merely that the child had become a zombie because
of missing waitpid() call.

mysql-test/lib/mtr_process.pl

@@ -770,7 +770,15 @@ sub mtr_record_dead_children () {
 }
 
 sub start_reap_all {
-  $SIG{CHLD}= 'IGNORE';                 # FIXME is this enough?
+  # This causes terminating processes to not become zombies, avoiding
+  # the need for (or possibility of) explicit waitpid().
+  $SIG{CHLD}= 'IGNORE';
+
+  # On some platforms (Linux, QNX, OSX, ...) there is potential race
+  # here. If a process terminated before setting $SIG{CHLD} (but after
+  # any attempt to waitpid() it), it will still be a zombie. So we
+  # have to handle any such process here.
+  while(waitpid(-1, &WNOHANG) > 0) { };
 }
 
 sub stop_reap_all {