tests: improve go-tuf testability

[rdimitrov]

Based by a comment from @znewman01 in https://github.com/theupdateframework/go-tuf/discussions/291

Right now we have a lot of "instantiate a thing, and then do a whole bunch of miscellaneous operations with it because instantiating requires so much boilerplate" tests, which often feels like flailing. I'd like to see that migrate towards many tests of specific scenarios/properties, with a purpose.

[znewman01]

We had a discussion about this in the go-tuf maintainer's meeting (everything following "The main way we test this is these big Python fixtures").