testsuite/a11y: Don't check minimum increment

testsuite/a11y: Don't check minimum increment

This value is highly sensitive to changes in the theme; we
don't want to have to tweak hardcoded integers in this test case
every time we rev gnome-themes-standard.

Future iterations could do more heuristic matching, but this
is enough for now.

https://bugzilla.gnome.org/show_bug.cgi?id=704747