Add tests specific to XWayland

[wismill]

Currently we only test Xorg server using xkbcomp, but XWayland requires to be tested slightly differently.

This is motivated by this comment:

I found my mistake: keymap received from Wayland is processed using XkbCompileKeymapFromString(), which then call XkbDDXLoadKeymapFromString() which finally calls RunXkbComp(). But this does not load the keymap in the server and instead creates a .xkm file. The call to XkbDeviceApplyKeymap() is where the server is updated.

[wismill]

Idea: use headless Weston

[wismill]

xwayland-run would be most useful, but there is an issue with all the headless compositors: there is no input by default nor an easy way to add some!

It seems for now a XWayland test can only be achieved by writing a minimal headless Wayland compositor with some dummy default keyboard.