Fix bug in dl-dt list output generation
* An open <dt> (;) should be closed when we encounter a new <dd> (:)
char even if it is on a new line that has other nested lists inside.
* Tidy was hiding this PHP parser bug by closing a <dt> and opening
a <dd> when given this HTML: "<dl><dt>a<ul><li>b</li></ul></dt></dl>"
It generates "<dl><dt>a</dt><dd><ul><li>b</li></ul></dd></dl>"
However, a HTML5 parser like RemexHTML, domino (used by Parsoid),
or browsers don't do this fixup.
* So, what I thought was a bug in RemexHTML turned out to be a bug
in the PHP parser that was being hidden by the use of Tidy.
* Added a regression test.
Bug: T175099
Change-Id: I6d5b225b82cecf9a43f23837ed8ec359b31aadad