test: attempt to send QUIT to smtp-dummy in case mail send failed

If mail sending from emacs fails before it has chance to connect
to the smtp-dummy mail server, the opportunistic QUIT message
sending makes smtp-dummy to exit.

test/test-lib.sh

@@ -398,6 +398,8 @@ emacs_deliver_message ()
 	   (insert \"${body}\")
 	   $@
 	   (message-send-and-exit))"
+    # opportunistically quit smtp-dummy in case above fails.
+    { echo QUIT > /dev/tcp/localhost/25025; } 2>/dev/null
     wait ${smtp_dummy_pid}
     notmuch new >/dev/null
 }