Bug 1654112 - Add Xrandr.h and XTest.h to system headers. r=ng

The Linux desktop capture code now relies on these. The build will fail at the
link stage if these are not in system header wrappers.

Differential Revision: https://phabricator.services.mozilla.com/D130090

config/system-headers.mozbuild

@@ -1030,6 +1030,7 @@ if CONFIG['MOZ_X11']:
         'X11/extensions/Xfixes.h',
         'X11/extensions/Xrandr.h',
         'X11/extensions/XShm.h',
+        'X11/extensions/XTest.h',
         'X11/ImUtil.h',
         'X11/Intrinsic.h',
         'X11/keysymdef.h',