Write tests to prevent unexpected changes in full.json or unexpected invocations in evergreen.yml from breaking multiversion tests. The self test should check that
a correct download is generated.

One way to do this is to test MultiVersionDownloader.download_links()
and ensure that it generates the correct download URLs for a different
combinations of cmdline arguments.

I've found Python's "mock" library helpful for overriding specific functions and
could be used here.