Draft: Running tests natively, using modules

This relies on the latest Catala version, including https://github.com/CatalaLang/catala/pull/502

It's intended as a demo of how we expect to run tests using modules. The manually-coded Makefile rules and the catala module are intended to be replaced by handling from clerk.

Current limitations:

• since Catala doesn't support cross-module disambiguation yet, all references in the tests had to be fully qualified (e.g. Impot_revenu.Enum.Case)
• module usage is fully on the command-line, not in the language
• since the tests code refer to the IR module, they can't be text-included in the main code (for inclusive literate output): the name would be unresolved.

I am not sure how best to avoid this last limitation ; one way would be to include all the test cases in the master file (without IR. qualification) and then have a test file that only has the catala-test-inline stanzas ; but that seems less convenient and error-prone.

Or maybe a clever module naming/aliasing/opening system could help.