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 release in 5 minutes and 50 seconds (queued for 23 seconds)
Status Job ID Name Coverage
failed #125071460


Name Stage Failure
test_release_branch Test
NameError Traceback (most recent call last)
<ipython-input-38-0ce74537a13f> in <module>()
----> 1"monolayer_distance_map0.tif", overwrite=True)

NameError: name 's_monolayer' is not defined

===================== 38 failed, 1 skipped in 4.25 seconds =====================
ERROR: Job failed: exit code 1