Add `crystal tool format` tests

[ysbaddaden]

I'm not sure all projects use the formatter. I mean that mine usually don't use it, so just checking for the return type may not be enough. For example ysbaddaden/minitest may only pass the formatter check by sheer luck :sweat_smile:

[straight-shoota]

crystal_format is an explicit test, so we can easily drop it on any project which doesn't maintain the format. We can do that as we notice breakages.