tests should skip if there's no $DISPLAY

If the bzr-gtk tests are run with no $DISPLAY set, we get a lot of warnings, followed by a crash (on Hardy):

/home/mbp/.bazaar/plugins/gtk/diff.py:71: Warning: g_object_unref: assertion `G_IS_OBJECT (object)' failed
  self.add(sourceview)
/home/mbp/.bazaar/plugins/gtk/diff.py:71: GtkWarning: gtk_settings_get_for_screen: assertion `GDK_IS_SCREEN (screen)' failed
  self.add(sourceview)
/home/mbp/.bazaar/plugins/gtk/diff.py:71: GtkWarning: gdk_screen_get_display: assertion `GDK_IS_SCREEN (screen)' failed
  self.add(sourceview)
/home/mbp/.bazaar/plugins/gtk/diff.py:71: GtkWarning: gdk_keymap_get_for_display: assertion `GDK_IS_DISPLAY (display)' failed
  self.add(sourceview)
/home/mbp/.bazaar/plugins/gtk/diff.py:71: Warning: g_object_get: assertion `G_IS_OBJECT (object)' failed
  self.add(sourceview)
zsh: segmentation fault (core dumped) ./bzr selftest
./bzr selftest 398.13s user 26.61s system 60% cpu 11:43.90 total

It would be nice if (on unix) these tests skipped if there was no X display. (I realize detecting whether it's valid will be hard, but just checking the variable is set should be enough.)