fix(color-contrast): correctly determine color contrast for element that exactly overlaps midpoint of node

[straker]

In https://github.com/dequelabs/axe-core/pull/2003/files we changed the algorithm from < to <= to handle nodes returning as incomplete in the code highlighting. The tests added for test/integration/full/contrast/code-highlighting.html assert that only 1 incomplete is returned, so this change does not affect the reason we moved from < to <= in the first place.

Closes issue: #3563