Fix backend tests failing if FIRST_SUPERUSER_PASSWORD is not "changethis"

[swelborn]

See discussion here: #1135 (reply in thread)

This was changed in the following PR: #632

Changing it back to settings.FIRST_SUPERUSER_PASSWORD makes it work again.

I think the issue is that after this test, the password is changed to something else and the other tests can't authorize.

There was another fix in that discussion here: #1135 (reply in thread)

Don't know what is preferred, let me know.

[swelborn]

Forgot to add...

To reproduce:

1. Clone master
2. Edit FIRST_SUPERUSER_PASSWORD in .env to something other than "changethis"
3. docker compose up
4. Exec into backend container
5. run bash ./scripts/test.sh