test: Rename util.c and util.h to util-x11.c and util-x11.h

These utility functions are all specific to the libX11 interface already,
and since we're planning to add some other utility functions soon, (such
as EGL), it will help to not have a too-generic name already used.

While doing this, also split up the interfaces for Display and Window
creation. This will allow us to create the GL context in between to
guarantee that the Window is created with the same visual as the GL
context.