Unit testing of method that return json encoded data in CodeIgniter

Can anyone suggest me any CI library for unit testing (with proper documentation) . Please don't suggest the codeigniter's default library for unit testing . I guess , it is not fully functional . I want to test my model which is returning json encoded data . For example here is my sample code .

http://ift.tt/1xTdiwz

Can anyone suggest me how to test this function using a unit testing library ? Thanks in Advance