#616 GTK3V2MAIN: PreferenceTest fails in CI — no display for GTK initialization

[Allen D Byrne]

PreferenceTest (preftest) initializes the wlib/GTK layer via wInitAppName(),
which requires a display. CI Linux runners have no X display, so the test
fails immediately with:

(preftest): Gtk-WARNING **: cannot open display:

Fix: install xvfb and run ctest under xvfb-run in the CI workflow so
GTK can initialize against a virtual framebuffer.

Branch: bug-616-linux-xvfb-preftest