Tests: Replace resource loader with AMD

:boom: WIP...

Pulll request's goal is to AMDify tests for 1.12.0 release and address part of http://bugs.jqueryui.com/ticket/10119.


Ok, this is what happened: I took the liberty to reduce `TestHelper` into `helper`. In the end, we have one only helper global. Do you want me to revert the renaming?