Some tests (particularly when there are schedules and shifts involved)
depend on generated usernames being unique (we randomly get [CI
errors](https://github.com/grafana/oncall/actions/runs/6812435056/job/18524768961#step:5:676)
because of this). Although username field is not required to be unique,
I think it makes sense for testing purposes to enable this (and when a
duplicated username is needed in a test, make that explicit).
e41ccb9d9b