test: Fix meson build with xvfb disabled.

Fixes #678
Signed-off-by: Eric Anholt <eric@anholt.net>

test/meson.build

@@ -36,14 +36,15 @@ rendercheck_tests = [
 ]
 
 rendercheck = find_program('rendercheck', required:false)
-xvfb_args = [
+
+if get_option('xvfb')
+    xvfb_args = [
         xvfb_server.full_path(),
         '-screen',
         'scrn',
         '1280x1024x24',
     ]
 
-if get_option('xvfb')
     test('XTS',
         find_program('scripts/xvfb-piglit.sh'),
         env: piglit_env,