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This fixes #48 and chooses Minitest as the test framework for sup.

Picking Minitest is pretty much a personal choice since:

* It doesn't load many files, making the TDD cycle pretty fast even on a spinning HDD.
* It provide a spec-like syntax while doesn't enforce weird matcher syntax.
* Easy to migrate from test/unit, which I already did in f14b03b

### Checklist

* [ ] Backport to line-0.13-develop