Switch lockscreen test to OpenGL

Summary:
Main reason for using QPainter was the fact that build.kde.org did not
support OpenGL back when the test got introduced. As we have vgem
support nowadays we can switch back to OpenGL.

Test Plan:
100% tests passed, 0 tests failed out of 130,
but the test is flaky with and without this change

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D16940