Implement a proper test suite

[mrkkrp]

This is necessary to move on with the development at this point.

[Monkeypac]

Ok so I'll start here.

By a test suite, you mean a lisp side test suite ? Or something that would check that inside a buffer, the folds a correctly folding / unfolding ?
The first one seems to be doable to me while the second one looks more like a continuous integration thing.

Maybe it would be great to have :

• a lisp side automatic test suite that would test:
  • folds / text properties
  • folds creation / deletion
  • anything I'm not thinking about
• something like test instructions for manually testing some features that would test:
  • fold creation / deletion
  • go to next / previous
  • refolding
  • options (like the fringe color, the fringe side, ...)

[mrkkrp]

I just merged #49, which should give you an idea what we could test and how.