This is to allow us to store new policy files in the production TUF repository
under a testing delegation, and for clients to opt-in to using this testing
delegation when retrieving policy from TUF.
If the prefix path is set, it is prepended to every target path on download
with path.Join. For example, if the prefix path is testing and we download
the target a/b, the TUF client with actually download testing/a/b.
Also get the latest testdata from tuf-dev.
