t/ckeditor5-media-embed/1: Made the view stringify util output the content of the UI element

[oleq]

Suggested merge commit message (convention)

Other: Made the view stringify util output the content of the UIElement (see ckeditor/ckeditor5#2727).

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A piece of ckeditor/ckeditor5-media-embed#2 constellation.

[coveralls]

Coverage increased (+0.0008%) to 98.617% when pulling 7801f21 on t/ckeditor5-media-embed/1 into ef5960e on master.

[Reinmar]

I'm against this change, exactly because of what you had to do in ckeditor5-table:

Want to check the content of a UI element? Check it (by accessing it and checking what its render() returned), but don't affect the entire stringify() method which then becomes far less useful for many other cases.

Alternatively, we could make this configurable in stringify(). But I've got problems believing in innerHTML's stability across browsers, so I've got mixed feelings whether it's safe at all.

[oleq]

I went for the config option which made the table PR obsolete.