Better test structure

[thumphries]

The tests currently involve a shell script defined only in the CircleCI configuration that builds a couple of random packages via direct workspace references, i.e. not how anybody actually uses Hazel.

A local_repository with own WORKSPACE or similar would be a nice improvement, especially if it allowed us to test multiple snapshots at once for some characteristic set of packages. Maybe best to borrow whatever structure is used in rules_haskell.