Fixed wording in unit tests documentation.

If you're using a backend that isn't SQLite, you will need to provide other

details for each database:

* The :setting:`USER` option for each of your databases needs to

  specify an existing user account for the database.

* The :setting:`USER` option needs to specify an existing user account

  for the database.

* The :setting:`PASSWORD` option needs to provide the password for

  the :setting:`USER` that has been specified.