Rewrite spec-example tests as testharness.js tests

Because of small differences in the way browser engines do math and
layout, it is very hard to come up with a reftest that renders the same
in all browsers. Switching to a testharness test with fuzziness allows
the tests to be much more robust.
Update radial gradient tests to use testharness

Layout methods differ slightly in their computations between
implementations, causing minor differences in the positioning due to
shapes. Use testharness instead of ref tests so that the tests can
account for these differences.
Merge pull request #637 from bemjb/gradient

Update radial gradient tests to use testharness
Remove forgotten prefix, add failure on timeout to address review
comments.
Merge pull request #635 from bemjb/spec

Rewrite spec-example tests as testharness.js tests