Unbreak test tools, allowing building with gcc-14. (Closes: #1075540)

The test-send-xmessage* tools were passing pointers (to argv) to interfaces that expect ints (32 bit IDs). This looks like it has never worked, but I don't know why nobody noticed earlier.

This has not been tested beyond building, but its potential impact is limited as it only touches testing tools that Debian already does not run or ship.

Forwarded as https://gitlab.freedesktop.org/xdg/startup-notification/-/issues/7