Moved the YUI sandbox into the describe closure
Moved the YUI sandbox into the describe closure

For whatever reason one of the describe closures in test_model.js
was wrapped in a YUI sandbox. This caused whatever test suite was 
run after it in .only mode to fail with an extend failure. Moving
the YUI sandbox inside to match our other tests has solved this.

R=gary.poster, rharding
CC=
https://codereview.appspot.com/8099047

bac/juju-gui,CanonicalJS/juju-gui,jrwren/juju-gui,jrwren/juju-gui,bac/juju-gui,jrwren/juju-gui,jrwren/juju-gui,CanonicalJS/juju-gui,CanonicalJS/juju-gui,mitechie/juju-gui,mitechie/juju-gui,bac/juju-gui,mitechie/juju-gui,mitechie/juju-gui,bac/juju-gui