Refactor tst_QAbstractSlider::keyPressed()

Don't do several tests at once in the test function. Instead, move
the extra tests to the data function. This makes it possible to easily
add a self-contained test (i.e row) for an upcoming fix.

Task-number: QTBUG-25988
Change-Id: I65c8d7620f01107f8f59c96896b1a641d97f5fdc
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>