test: Fix meson build with xvfb disabled.

Fixes #678

Signed-off-by: Eric Anholt <eric@anholt.net>
3 jobs for test-fix in 7 minutes and 27 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #215483
debian-testing

00:00:31

 
  Build And Test
passed #215484
autotools-build-and-test

00:06:55

passed #215485
meson-build-and-test

00:02:08