Test GLX and EGL-X11 meaningfully
This is sort of the converse of testing this in Mesa's CI. Here we would select known versions of mesa and piglit (or whatever test suite), and use that to verify that xserver changes don't regress.
There are no less than six GLX backends we want to test here: software DRI (Xvfb), hardware DRI (Xorg), EGL (Xwayland post-1.20), glxproxy (Xdmx), Xwin and Xquartz. Sure would be nice to chop that number down a bit.