tests: Move `saveload` utility to tests.game

Hopefully easier to reuse it in other tests this way.
tests: Add `saveload` utility for gui tests as well

As there are differences between the game test version and the gui test
one, having both around where they belong probably makes more sense than
one combined helper.
tests: Use `saveload` in boat builder test
tests: Save and reload tutorial during test progression

This should cover some basic scenario save/load functionality.