Updating addUser util with more functionality

/test/utils/addUser.js

1. Currently only returns a new user created using the index 0 of the /test/fixtures/user/user.js

In cases where we need multiple users, if we call the addUser() multiple times, we will not add a new user and instead update it.
This might be an issue for tests where we need to use multiple users.

2. In cases we where need to create a super_user (authorized user) we do not have any functionality for the same as well

Currently, we are achieving this by importing the fixtures and picking the required user index from them and passing that as an argument to addUser()

Suggestion: We can create another util function in the addUser.js called addSuperUser() to solve issue 2 mentioned above.
As for issue 1, we will need to decide do we need a new solution or do we stick with the current method

Start with /test/utils/addUser.js and the user fixtures at /test/fixtures/user/user.js. Check how repeated addUser() calls select and persist users, and how tests currently create super users. Done means the utility supports distinct users for repeated calls and provides the proposed super-user functionality, with the chosen behavior documented by the relevant tests.