Get go-tuf client upstreamed or remove it

[jku]

Maintaining the go-tuf test client is not a vast amount of work but it is work. If the upstream project is not interested in testing with conformance, I don't think I'm interested in maintaining the test client here.

I would prefer it if

• go-tuf project would start testing with conformance and would maintain the test client in the go-tuf sources
• we can copy paste the test client from there (this is how python-tuf currently works)

The amount of work remains the same but
a) the work mostly happens in go-tuf project
b) go-tuf actually gets tested with tuf-conformance

[jku]

theupdateframework/go-tuf#649