New review comment by paper42 on void-packages repository

https://github.com/void-linux/void-packages/pull/32005#discussion_r684656460

Comment:
we can drop _xdg_runtime_dir, pre_check and post_check if we add
```
do_check() {
	mkdir /tmp/gnome-shell-xdg
	XDG_RUNTIME_DIR=/tmp xvfb-run ninja -C build test
	rm -rf /tmp/gnome-shell-xdg
}
```