veyron/services/mgmt/device/impl/impl_test.go: test helper refactoring
veyron/services/mgmt/device/impl/impl_test.go: test helper refactoring

Refactor verifyAppWorkspace to permit easy implementation of a test
helper to check on the state of an application instance.

Change-Id: Id6f2315e626ac1d9cdd48671bb9500890b7b5dfb

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref