a testing `feature`?

[skaunov]

Consider adding a feature for the testing code.

The arbitrary feature was unmaintained (IIRC build was failing when only it was selected), so it would make sense to "merge" those and have one feature for testing and arbitrary needs.

Such feature itself would straighten up two things. There's some code for integration testing which is now just there hidden from the docs. And contrary the traits like ConsensusProgramSpecification are strictly omitted by the docs compiler, and there's no other reason to promote that code from behind #[cfg(test)], but having it in the overall docs would be still very useful.