Reconsider the diff config option

[sampsyo]

We introduced a diff option to control what happens when you do turnt --diff. But I regret putting this in turnt.toml because it is more about the user's interaction preferences, not the setup for the tests themselves. Maybe we should just get rid of this and make it something sensible by default. (Or conceivably we could invent some sort of user config file, but that seems annoying to do…)

[tekknolagi]

You could do difflib if --diff is passed or if it's given a value with --diff=... then run that instead (or --diff-cmd as a separate thing is probably cleaner)