Test document fragment handling in Danger.dangerouslyRenderMarkup.
Allow Danger.dangerouslyRenderMarkup to return document fragments.

A document fragment will be included in the returned array for any markup
that renders as more or less than one node.

This deviation from typical one-to-one rendering previously caused an
invariant failure, so no code that previously worked will now be broken,
and most code that was previously broken because of unexpected parsing
behavior will now just work.  Any code broken as a result of this commit
probably just needs to add some logic to handle document fragments.  But
hey, we didn't call it `dangerouslyRenderMarkup` for nothing.