Tests correct button behavior with respect to collapsing and baseline: Empty buttons have a baseline at the bottom of the content box; buttons with text are aligned so that the text lines up. Also, empty buttons should collapse.

To understand this test, note that with the Ahem font, the bottom of the "X" is 2px below the baseline (at a 10px font-size).

 X   X

PASS
PASS
PASS
PASS