editoast: add test coverage to `GET:/timetable/{id}/requirements`

[Sh099078]

Description and goal

The endpoint is not tested. Add a few tests about its expected behavior and its authorization.

Acceptance criteria

Tests exist about:

• Its general behavior: what payloads it accepts and what it returns.
• Its authorization: which roles and grants are needed to use it without getting a 403 Forbidden response.