Investigate e2e with supertest (or equivalent)

[GuillaumeDeconinck]

Currently the e2e run as two containers, the API and the tests-runner. The API is the release Docker image (no dev-dependencies). While it's good to test the actual release Docker image, in the long run it can take a long time to bootstrap and execute. Using supertest (or equivalent) might help to speed up the tests.

Of course for the moment, the time taken to run the e2e tests isn't an issue (yet).

The solution might be to have both

• E2E tests with supertest => faster with more coverage
• E2E tests like now, but just a few to be sure the API runs correctly (but not checking all the product requirements)