MDEV-28976 fixup: A better fix

do_shutdown_server(): After sending SIGKILL, invoke wait_until_dead().
Thanks to Sergei Golubchik for pointing out that the previous fix
does not actually work.

client/mysqltest.cc

@@ -5191,9 +5191,10 @@ void do_shutdown_server(struct st_command *command)
     if (timeout)
       (void) my_kill(pid, SIGABRT);
     /* Give server a few seconds to die in all cases */
-    if (wait_until_dead(pid, timeout < 5 ? 5 : timeout))
+    if (!timeout || wait_until_dead(pid, timeout < 5 ? 5 : timeout))
     {
       (void) my_kill(pid, SIGKILL);
+      wait_until_dead(pid, 5);
     }
   }
   DBUG_VOID_RETURN;