gitlab-ci: remove xvfb-run for script testing due to Segmentation fault

For some reason, calling a plotting function, then exiting led to a
Segmentation fault. This then caused the tests to fail.
1 job from master in 16 minutes and 46 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #125006295
test_non_release_branches

00:16:46