✅(backend) fix flaky tests

[jonathanreveille]

Purpose

We have a flaky test (test_admin_api_user_list_filter_by_id) on the CI where the results are not sorted the expected way. To have more consistency overtime, we have decided to sort the result of the list endpoint by username.

We also have another flaky test (test_api_admin_orders_course_retrieve) that happens rarely, it's with the OrderGroup factory. If we don't set a number of seat, it can choose a number between 0 to 100 for this value. The issue is that when it sets 0 for the number of seats and we try to create an order on that order group, it would block the creation. To avoid this behavior, we have decided that the factory can choose a value between 1 to 100 if no value is setted during creation.

Proposal

• [x] Add ordering by username for admin api users viewset
• [x] Change the range of the random number of seats of the OrderGroupFactory when not setted in creation