docs: The X Test Suite uses meson [skip ci]

docs/markdown/Users.md

@@ -132,6 +132,7 @@ format files
  - [wxFormBuilder](https://github.com/wxFormBuilder/wxFormBuilder), RAD tool for wxWidgets GUI design
  - [xi-gtk](https://github.com/eyelash/xi-gtk), a GTK+ front-end for the Xi editor
  - [Xorg](https://cgit.freedesktop.org/xorg/xserver/), the X.org display server (not the default yet)
+ - [X Test Suite](https://gitlab.freedesktop.org/xorg/test/xts), The X.org test suite
  - [zathura](https://github.com/pwmt/zathura), a highly customizable and functional document viewer based on the
 girara user interface library and several document libraries
  - [Zrythm](https://git.zrythm.org/cgit/zrythm), a cross-platform digital audio workstation using GTK+ 3