tests: Run tuf-conformance tests

[jku]

tuf-conformance is a test suite for TUF clients. It should help with spec conformance as well as practical compatibility with other client implementations.

• It would be useful for go-tuf to run the test suite in CI (definitely for PRs but maybe also in cron)
• The README has the instructions for setting this up -- in go-tuf case it's very easy since there is a client-under-test implementation already in the test suite: I suggest copying a version of that into your sources though.