[OAuth] Implement functional tests

[netsensei]

Functional testing for OAuth has been implemented partially. Currently, only the conumser workflows are tested. We need to expand this to the other roles: administrator and manager.

Detailed description

Implement these functions:

• testManageClientsAsAdministrator
• testManageClientsAsManager

Take time to consider other test scenario's i.e. deleting clients and associated access and refresh tokens.

Context

Functional testing is important to ensure the functional integrity of the application (do we cover all the necessary use cases? Functional testing forces us to consider us to take time and think before we write code.