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.
2 jobs from release in 5 minutes and 41 seconds
Status Job ID Name Coverage
  Test
failed #125012983
test_release_branch

00:05:41

failed #125009112
test_release_branch

00:03:28

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

NameError: name 's_monolayer' is not defined

===================== 38 failed, 1 skipped in 4.23 seconds =====================
ERROR: Job failed: exit code 1