[GTK] run-gtk-tests randomly fails while running the xprop comand

[GTK] run-gtk-tests randomly fails while running the xprop comand
https://bugs.webkit.org/show_bug.cgi?id=76817

Reviewed by Gustavo Noronha Silva.

No need to use xprop to remove the AT_SPI_BUS property since
run-gtk-tests will always run new instances of Xvfb.

* Scripts/run-gtk-tests:
(TestRunner): Unskip TestWebKitAccessibility.
(TestRunner.run): Uncomment lines for launching the accessibility
bus and registry daemon, and remove lines for running xprop.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105617 268f45cc-cd09-0410-ab3c-d52691b4dbfc