Make the cli of openfisca test, testable

[bonjourmauko]

Hi there!

I really enjoy OpenFisca, but I recently encountered an issue.

Here is what I did:

I've been adding test, types, and documentation to several modules.

Here is what I expected to happen:

To fix bugs and improve code resilience.

Here is what actually happened:

Unsuspected bugs are discovered or introduced when working with openfisca test and tracers.

Here is data (or links to it) that can help you reproduce this issue:

See for example all the issues listed in #1309