Fix error at shutdown on WorkManager in DAM tests

2014-09-08 15:20:34,020 ERROR [localhost-startStop-2] [org.nuxeo.runtime.model.impl.ComponentInstanceImpl] Failed to deactivate component: service:org.nuxeo.ecm.platform.video.service.VideoService
java.lang.IllegalStateException: Work manager not started, could not access to executors
	at org.nuxeo.ecm.core.work.WorkManagerImpl.getExecutor(WorkManagerImpl.java:310)
	at org.nuxeo.ecm.core.work.WorkManagerImpl.shutdownQueue(WorkManagerImpl.java:328)
	at org.nuxeo.ecm.platform.video.service.VideoServiceImpl.deactivate(VideoServiceImpl.java:79)
	at org.nuxeo.runtime.model.impl.ComponentInstanceImpl.deactivate(ComponentInstanceImpl.java:142)
	at org.nuxeo.runtime.model.impl.RegistrationInfoImpl.deactivate(RegistrationInfoImpl.java:429)
	at org.nuxeo.runtime.model.impl.RegistrationInfoImpl.unresolve(RegistrationInfoImpl.java:462)