Externally publish JUnit->Truth migrations

RE: https://twitter.com/foobarjet/status/1148700099331219456

We have these internally (in `TruthExhaustiveShortcuts` and scattered through the classes in the `truth` subpackage), but we haven't published them for external users.

I believe that they all use [Refaster](http://errorprone.info/docs/refaster), which is available externally. (I see that [we've been slow to publish other Refaster templates](https://github.com/google/error-prone/issues/649), but hopefully the Truth-specific ones are both easier and more valuable than average.)