Use url attr to get the url of the media used by cover block

[matiasbenedetto]

What?

• Updates how the linked media gets from the core/cover block.
• Add a test case for cover blocks using background images.

Why?

Fixes #748

How?

It no longer parses the block's HTML but relies on its' URL' attribute.

Testing instructions:

• Add a cover block in a template using the site editor with an image from the media library.
• Enable the 'Fixed background' or 'Repeated background' option in the cover block styles sidebar.
• Save the changes in the editor.
• Save the theme using CBT with the 'Localize Images' option enabled.
• Check the code of core convert in the resulting pattern. It should reference an image from the theme with a relative URL, not the media library URL.