[autotests] Fix LockScreenTest::testKeyboardShortcut

32f4e115e2c68b75a5eea86e0a582388d5abadb9 broke the test. To my
surprise there was a test using the kwin internal shortcut handling
instead of using KGlobalAccel.

This change adjusts the test to go through KGlobalAccel for testing
the global shortcut.