expose RefTime in API for testing

[jku]

A go-tuf client uses TrustedMetadata.RefTime as the clients refresh reference time: normally this set to time.Now().UTC().

For testing purposes it would be useful if RefTime could be set to other values. tuf-conformance tries to handle this by using faketime but this does not work with golang (since golang does not use the system calls that faketime changes with LD_PRELOAD): theupdateframework/tuf-conformance#263. I believe the only way for tuf-conformance to be able to do this is to either:

• change actual system time -- this is not a good idea in a test suite
• have some sort of ability to modify RefTime via the API -- or maybe this exists already somehow?

[jku]

oh! Updater.UnsafeSetRefTime() looks very promising... Maybe this only needs some work in the test client (which currently lives in tuf-conformance but should be in this repo #649 )

I'm going to close this as that looks like it will work