Re-organize test/sampler.jl

[mtanneau]

We are starting to saturate test/sampler.jl, which currently includes unit tests for the data sampling code, and functional tests for the data-generation (OPF sampling and solving).

It would be good to separate those.
On top of my head:

• tests for update! of OPF models should go with their respective formulations
• tests for data distributions and sampling should be isolated, and preferably moved to a test/sampler folder (mimicking the structure of the source code)
• tests for data generation (sampling and solving OPF instances) should be isolated too. It's less clear to me what this should look like.
  I'm thinking we might want to move code currently in exp/sampler.jl to source code.

[klamike]

Ideally we can address #99 together with this (when moving exp/sampler.jl to source)

[mtanneau]

I'll get started on moving exp/sampler.jl to source.