Extend unit tests

[eseiver]

Right now, the unit tests do simple transformations from DOIs to URLs to file paths. What might be other unit tests to include? Ideas:

• whether the example url links to XML are valid & the downloaded file is valid
• whether an example solr query returns expected results
• whether XML parsing functions work as expected

Need to also allow people to test the corpus even if they haven't downloaded the corpus and if they don't have internet access. This should involve a very small directory of testing files included in the repo