Add tests for background-clip: text with multiple background layers

[johannesodland]

There are a few tests for background-clip with multiple image layers. Most of these test clipping of background color:

• background-clip-root.html Background of root.
• background-color-border-box.htm Clip of background color.
• background-color-clip.html Clip of background color.
• background_color_padding_box.htm Clip of background color.

Unfortunately, there don't seem to be any tests for background-clip with multiple image layers where the top most layer is clipped to text. Firefox has an open issue on this.

There's a need for better test coverage in this area. Test should include multiple image layers, and test combinations of background-clip, including:

  background-clip: text, content-box, padding-box, border-box;

Note while background-clip: text is only specified in the css-backgrounds-4 draft, the value has been supported in most browsers since 2016.