gnu: Use the test-only Xorg variant for tests.

gnu: Use the test-only Xorg variant for tests.

* gnu/packages/education.scm (gcompris-qt)[native-inputs]: Change XORG-SERVER
to XORG-SERVER-FOR-TESTS.
* gnu/packages/engineering.scm (pcb, qucs, openscad)[native-inputs]: Likewise.
* gnu/packages/gnome.scm (libbonoboui, d-feet, gspell, libdazzle, libhandy)[native-inputs]: Likewise.
* gnu/packages/gnupg.scm (parcimonie)[native-inputs]: Likewise.
* gnu/packages/graphics.scm (opensubdiv, rapicorn)[native-inputs]: Likewise.
* gnu/packages/gtk.scm (gtksourceview-2, gtksourceview)[native-inputs]: Likewise.
* gnu/packages/image-processing.scm (opencv)[native-inputs]: Likewise.
* gnu/packages/kde-frameworks.scm (kconfig, kcoreaddons, kwidgetsaddons,
kwindowsystem, kdeclarative)[native-inputs]: Likewise.
* gnu/packages/kde-plasma.scm (kscreenlocker)[native-inputs]: Likewise.
* gnu/packages/messaging.scm (gajim)[native-inputs]: Likewise.
* gnu/packages/python-xyz.scm (python-seaborn)[native-inputs]: Likewise.