test: Increment mtime when delivering a message with emacs_deliver_message

Without this, mail messages delivered by emacs_deliver_message might
not be seen by the next invocation of "notmuch new", (which can lead
to test-suite failures if emacs_deliver_message is fast enough).

test/test-lib.sh

@@ -409,6 +409,8 @@ emacs_deliver_message ()
 	   $@
 	   (message-send-and-exit))" >/dev/null 2>&1
     wait ${smtp_dummy_pid}
+    increment_mtime "$MAIL_DIR"/sent/cur
+    increment_mtime "$MAIL_DIR"/sent/new
     notmuch new >/dev/null
 }