`linear_gradients_lengths_ref.html` use `background-clip: content-box`.

The tests clearly expect the background painting area to have width of
100px, but since the default value of `background-clip` is `border-box`
and the boxes in question have a 1-pixel border the background painting
areas are actually 102px. This commit fixes the tests so that the width
of the background painting area is actually 100px.