Add remaining function tests for UserManager and BaseManager to validate concept #2073
Description
Add new test files to validate the YAML function test concept for manager classes:
- Add test_usermanager.yaml to cover all zhmcclient API methods and properties of the zhmcclient.UserManager class that are defined there.
- Add test_manager.yaml to cover all zhmcclient API methods and properties of the zhmcclient.BaseManager class.