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

For now, I think we can create a function that gets the user connection info from an  environment variable.  For example:  RA_TEST_USER=username=foo;password=bar. We can then have function Get-UserInfo  that for now creates a PSCredential from the environment variable, whcih we can use to log in., this would change the signature for this function just abit, and make it like:
$userCreds= (Get-UserInfo)
Create-Ra $userCreds.username <scope> <permissions>
Test-RaUserPermissions $userCreds '*/reader'