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

testsuite/a11y/range.txt

@@ -28,7 +28,6 @@ window1
       minimum value: 0.500000
       maximum value: 99.500000
       current value: 45.500000
-      minimum increment: 0.500000
     scale1
       "slider"
       parent: grid1
@@ -43,4 +42,3 @@ window1
       minimum value: 0.500000
       maximum value: 99.500000
       current value: 45.500000
-      minimum increment: 0.500000