In the spirit of "if you have to do something 3 times then factor it
out", make a testutils package to put all the `RoundTrip` boilerplate.
I don't overly like having test packages, especially mixed in with
code, but I don't see a nicer way of doing this without ending up
with a sprawling mess of copypasta'd test boilerplate which will
be an absolute nightmare to maintain.
I think this is the lesser of two evils.