Delete kglobalshortcutsrc before executing a test

Summary:
When running the complete test suite the ScriptedEffectsTest::testShortcuts
registers the shortcut meta+shift+y. But GlobalShortcutsTest::testX11ClientShortcut
also tries to use this shortcut for a window shortcut which fails if it
is already registered. So when running the complete test suite it
depends on the order of execution whether the GlobalShortcutsTest passes
or not.

Test should be clean - also the globalshortcuts. As the config is read
from test directory anyway, we can delete the file prior to init of
kglobalaccel.

Test Plan: Run ctest, test did not fail anymore

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D16932