Add endpoint that generates OpenFisca-compatible test files

[guillett]

Depends on #127.

[guillett]

@MattiSG I have done a few additional iterations on that PR to use it to regenerate tests for OpenFisca France (after the refactorings).

With the additional commits, YAML files that are generated work with an OpenFisca France instance with the following extensions:

• https://github.com/sgmap/openfisca-cd93
• https://github.com/sgmap/openfisca-paris
• https://github.com/sgmap/openfisca-rennesmetropole

(https://circleci.com/gh/guillett/openfisca-france/198)

[MattiSG]

The linked build does not describe where the tests are coming from, but if you're confident let's proceed :wink: If you need an explicit review, use the “request review” feature 🙂