Run the tuf-conformance test suite for go-tuf

[rdimitrov]

This is something that would be quite nice to have and will help continuously ensure go-tuf is conformant with the rest of the implementations.

References:

• https://github.com/theupdateframework/tuf-conformance
• Publish a conformance report tuf-conformance#322 (comment)

[jku]

The conformance README has the instructions for setting this up -- in go-tuf case it's likely 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