Move testing to minitest

[coderhs]

I feel the spec library of crystal is really limiting. As for example I am declaring the array variable on each case, instead of having a before_each. Guess I can blame my testing background for rspec and minitest for that 😬

I suggestion https://github.com/ysbaddaden/minitest.cr as it has the features like let or we can use spec2 (https://github.com/waterlink/spec2.cr) which has stuff like random order, before for current and nested context.

[werner]

spec2 doesn't look like to be maintained, I could consider migrating to minitest, but it seems to require some work.

[coderhs]

I can start work on it on another branch. Should be easy to implement or we can have both in the same project for now and then phase our spec, since minitest will be a development dependency I believe it won't be added when someone uses this shard.