Exposes mixin() method on test cases and test templates

Hello @jyhsu, 

Please review the following commits I made in branch 'dpup-mixin'.

f932fca4b368a621a8492f2b43a6dfd9f951106f (2014-05-09 07:54:18 -0700)
Exposes mixin() method on test cases and test templates

R=@jyhsu