testharness.js v. reftests for rendering tests

Hi,

As far as I'm aware, we have no real defined policy as to how rendering
tests should be written. We essentially have two options: testharness.js
using the CSSOM or reftests.

I believe the current Blink policy is to use the former except when
testing paint code, and Gecko's is to use reftests for both.

On the whole, despite the performance penalty, I'd much favour
recommending reftests for both given the intrinsic link between
rendering and painting and the various optimisations different
implementations do to avoid invoking the various parts with different
mutations.

Of course, if someone thinks the performance penalty is too high maybe
we'll have to reconsider.

/gsnedders