Fatal IO error 11 (resource is unavailable) on X server :2821 if Beremiz tests are running in xvfb using pytest

[Andrey Skvortsov]

look at tests/tools/conv.py

Something seems to be broken in Beremiz application,
because after tests in test_application.py during Xvfb shutdown
pytest returns error message:
pytest: Fatal IO error 11 (Die Ressource ist zur Zeit nicht verfügbar) on X server :2821.
#
As a result of that pytest returns code 1 as some tests were failed,
but they aren't.
#
To avoid this Xvfb is launched and killed not by pytest.
$ Xvfb :42 -screen 0 1280x1024x24 &
$ export DISPLAY=:42
$ pytest --timeout=10 ./tests/tools
$ pkill -9 Xvfb