Improve create_testdata so it doesn't break even when someone is actively using the frontend

create_testdata breaks mid-execution if there is a user in the User table who wasn't added by the test data. This can happen if someone is actively using the frontend while creating test data (since frontend requests trigger an addition of a user row if it doesn't exist already).

Because of this problem, ComSol's project README also warns that you should stop the frontend process entirely when generating testdata.

Turns out, this is quite inconvenient. I've hit this footgun multiple times locally when forgetting to stop the frontend. It also affects preview deployments, if users visit their deployment while there is create_testdata running.

Thus, it would be nice if we can modify create_testdata so it works fine even if there is a frontend running and there are users logging in.