Failing-er test for #2611

- Widths were harded to browsers that show scrollbars. If scrollbars are
  hidden, the test wouldn't fail.
- Added child elements for checking that the measure div is too _wide_ (on
  top of the existing children that help validate it isn't too _narrow_).
Fixes #2611 - Set parent width for coverGutter widgets in measure div

When using the measure div to measure a LineWidget with coverGutter: true,
set the parent's width to include the gutter. Otherwise, the widget will
be measured as if it were not meant to cover the gutter (it will be
measured at a narrower width).