What the above mean is that Policy Server will take the calculated Value of "testAttribute" and perform a String compare to see if it matches your condition 'Admin Group' If it matches, then 'Full_Admin' will be returned in the Attribute and if not , 'User' will be Returned
4) Now having said so, the calculated Value of your Custom user Attribute is "CN=Admin Group,DC=smtestenv,DC=com" which does not match the Condition 'Admin Group' when String compare is performed hence the issue you are having
To resolve the issue , you will need to Modify your Assertion Attribute Expression to have the Full DN of the Group as such
Once done, if the user belongs to the Group "Admin Group", then the calculated Value for testAttribute will be 'CN=Admin Group,DC=smtestenv,DC=com' , if not, it will be empty The policy server will then perform a String compare and if it matches the condition, the Full_Admin will be returned otherwise User will be returned