Improve tests

[ludovicm67]

We need to add missing tests and improve the code coverage to be sure things never breaks in case of a new release.

The work on having everything on a single repository was the first required step for it. Since it's mostly done, here are the next actions:

• add missing tests and check coverage
• uniformize tests
• create a support package that can be used in all tests ; this can help to generate a Trifid instance for the tests
• centralize dev dependencies at the root level, so that we have something easier to maintain