Make tests green on 20

This mostly involved using minutest and removing some deprecated assertions.

I did have to make one change to Process#file_writable? Some combination of the at_exit hooks installed by god, minitest and redefinition of at_exit by Minitest was causing the helper process to have an exit status of 1, even though exit(0) was called.

I don't think this changes the behaviour of the code, but then I'm not overly familiar with it - just trying to get a green test suite so that I can work on compilation failures on 2.2