This has to cheat and use a screenshot but thanks to the "Take Full
Screenshot" feature of Ladybird, it is very easy to update this test.
The steps are documented in the test.
We now keep the color value as a StyleValue up until we go to paint the
gradient, which makes `currentColor` work, along with any other color
values that can't be immediately converted into a `Gfx::Color` while
parsing.
The ref tests runner takes screenshots of both the input page and the
expected page, then compares them. Ref testing allows us to catch
painting bugs, which cannot be detected with the layout and text tests
we already have.
With ref tests, we'll likely want to reuse the same expectation page
for multiple inputs. Therefore, there's a `manifest.json` file that
describes the relationship between inputs and expected outputs.
