Make sure reftests aren't passing because nothing == nothing

In the test harness, assert that the two images aren't all white.

Fixes #3481
auto merge of #3645 : gilles-leblanc/servo/issue-3481, r=kmcallister

In the test harness, assert that the two images aren't all white.

Fixes #3481
layout: Rewrite intrinsic inline-size and automatic table layout to
match L. David Baron's work-in-progress specification.

    http://dbaron.org/css/intrinsic/

Column spans are not yet supported.

This effectively adds support for percentage widths, and it also fixes
many bugs, improving the layout of Google and Wikipedia.