[RBAC] Add mock tests for role assignment cmdlets (2265384)

I would rather wait a day or two to get Rbac functionality (which is under development now) and change it here than add a env variable. I see several problems in env. variables:
1. Add env variables to the build machines. and then we need to remove it back in a couple of days.
2. Change setup scripts in the tools folder to setup that user name and password (or at least ask for it). 
3. if the script will ask for username and password of the user under test, how do we pass that info between peoples who want to fix some breaks/tests and need to record mocks again? 

Let me know if you have any other concern.