[autotests] Fix failing KeyboardLayoutTest::testNumLock()

Summary:
Since Qt::KeypadModifier is set only for keypad keys and not the NumLock
key, we need to press at least one keypad key to determine whether
numlock is actually on. On the other hand, we know that when numlock is
on, the corresponding LED is also on. So we could check the LED rather
than press two keys.

Test Plan: testKeyboardLayout passes.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27789