Add failing test for empty lines in <string>

**do not merge, only failing test for now**

This is just a failing test to demonstrate a problem I had reading dayone plist
entries. The <string> values there are markdown, so empty new lines are important
because the denote paragraphs etc.

I tried tracing the error as best I could, and ended up in
`Mac::PropertyList::Source::get_line`, which doesn't return empty lines back
to the caller `read_next`.