fix hangs on mouse selection in case if wxPython uses wxWidgets with GTK3+ support

This is default configuration on Debian Buster+ and Ubuntu 18.04+.
Probably it'll fix completely problem with 'clipboard already open'
described in the previous commit 5ce6d08f ("make clipboard open minimal time as wxPython documentation recommends")

#!/bin/sh

LC_ALL=ru_RU.utf-8

export DISPLAY=:42
Xvfb $DISPLAY -screen 0 1280x1024x24 &
sleep 1

ret=0
DELAY=400
KILL_DELAY=$(($DELAY + 30))
timeout -k $KILL_DELAY $DELAY pytest --timeout=10 ./tests/tools
ret=$?

pkill -9 Xvfb

exit $ret