[autotests] Add test case for modifier only trigger when capslock enabled

This extends the testCapsLock to ensure that modifier only shortcut
trigger when capslock is enabled. So far the test only ensured that
capslock doesn't trigger the shift modifier only shortcut.

As the test shows currently when capslock is on, the meta key doesn't
trigger any more.

This test case goes into Plasma/5.9 branch and not in Plasma/5.8 as a
fix will most likely need the new xkbcommon 0.7 code paths which we do
not have in Plasma/5.8.

CCBUG: 375355