Fix running Xvfb

t.knopffrequested to merge
fix_compilation_issues into develop

This MR contains a fix for the Gitlab runner under Linux: for running the tests, the tool xfvb-run is used to simulate an X display for the GUI programs. The option -a has been added for automatically choosing a free display number since it seems without the option, always the same number is tried to be used. Probably hung build processes may block the reuse of that number.

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