Add a pantry test suite

[snoyberg]

API is stabilized enough that adding a test suite makes sense. This will give better feedback than relying on the Stack integration test suite to find bugs. CC @qrilka

[qrilka]

@snoyberg what's the scope of this ticket? I mean is 22c6464 already enough or the idea is to get good coverage of pantry code?

[snoyberg]

I'd say let's leave it open a bit longer and expand the test suite. I already spoke with @NorfairKing about lending some advice there.