[autotests] Fix KWindowInfoX11Test::testState

Summary:
KWindowInfoX11Test::testState(skipSwitcher) fails because
_KDE_NET_WM_STATE_SKIP_SWITCHER is not implemented in Openbox.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D19536