Fix CI test runs on stable

[matrss]

The CI test runs on stable fail consistently right now. Looking through the list of installed packages I saw that pyvirtualdisplay is now missing in the list for the recent runs of the tests. This might be the reason for the failures.

[matrss]

I guess the issue is that the installation of pyvirtualdisplay was removed from the Dockerfile that builds the image for stable, but stable still needs it (develop doesn't). It was removed here: Open-MSS/dockertesting@753d79a#diff-dd2c0eb6ea5cfc6c4bd4eac30934e2d5746747af48fef6da689e85b752f39557L42

The workflow should probably install it into the environment until it is no longer necessary, or just use xvfb-run like on develop.