refactor: Oauth20 tests
refactor: Oauth20 tests

Use App client app directly instead of requests. This removes dependency
on needing a web server running for your tests. Also, contributes to
coverage now. We can see which lines are impacted with each use case.

frappe/frappe,StrellaGroup/frappe,StrellaGroup/frappe,frappe/frappe,frappe/frappe,StrellaGroup/frappe