The existing content_tag tests document how to use content_tag outside
of erb views with data attributes. But for those who need to use
content_tag with nested content_tags and data attributes outside of
erb views, it can be challenging to find out how to do that.

When I had to do this in a project, it took me longer than I wanted.
Hopefully adding this test with an explicit example of how to achieve that
will others some time.