Missing test for the CSSColor class in css-typed-om/stylevalue-subclasses

In [css-typed-om/stylevalue-subclasses](https://github.com/web-platform-tests/wpt/tree/master/css/css-typed-om/stylevalue-subclasses) there are tests for all classes except for the [CSSColor](https://drafts.css-houdini.org/css-typed-om-1/#csscolor) class.
