Add test to check if a superuser can list all user accounts

[elyezer]

As a Cloudigrade QE, I want a test that checks if a superuser can list all created user accounts.

Acceptance Criteria

• Verify that there is a test that uses a super user to list all user accounts
• Verify that the test creates some user accounts
• Verify that the test asserts that all created user accounts are listed.

Assumptions and Questions

• API to see list of users (as superuser only) cloudigrade#279 is done