Speed up test setup using import endpoint

[ptitFicus]

Current backend / frontend tests builders intialize test situation using unitary endpoints (as if situation was set up through backoffice). This is kind of slow, for instance to create a project with 3 features, it needs:

• 1 call to project creation endpoint
• 3 calls to feature creation endpoints

This is not optimal, a best way would be to create an in memory ndjson file and import it, needing a single http call.