Add test-prep to the build distfile step
Add test-prep to the build distfile step

- The charm needs to build test_startup so that CI can work.

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https://codereview.appspot.com/9662049

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