Add optional test fixtures that generate predictable data.

Generate:

- users named `user0`, `user1`, etc. with password `aaaaaaaa`
- 3 projects for the 5 first predictable users, each with a visibility level

Development env only.

Can be turned on/off via an option (off by default), since the data so generated is inadequate to end user demos and snapshots.

I did not edit the CHANGELOG since I am not sure if such a minor dev only change should be mentioned there.