[JUNIT] ensure that all packet listeners are removed from the receiver

The receiver is not a session scoped component and so "lives" as long as
the plugin is loaded. Modified the test to make sure that all session
components deregistered their packet listeners.


Change-Id: I714ea1c1b34c65a86450e13fbd05a2101dbe92bb
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1135
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>