Added target_url key to tests and corresponding test (#171)

This PR addresses Issue #171.

I added the `target_url` key to the tests dictionaries and wrote a corresponding test for it. This was only necessary for the ReleaseScraper tests as the DirectScraper tests already check `final_url`

Please note, that I had to move the tests inside the test method in order to be able to make use of `self.wdir`. If this is undesirable, please comment.