temporarily allow prefiltering test mode to be configured via an osgi config

The prefiltering feature (of ChangeProcessor) currently has a System.property "oak.observation.prefilteringTestMode" which allows turning on/off a so called test mode. In the test mode prefiltering is evaluated but its result is not applied. Instead of applying, the prefiltering result (include vs exclude) is compared to whether any event is actually delivered to the listener. If an event is delivered and prefiltering result is exclude then a log.warn is issued.

While generally the System.property mechanism is slightly suboptimal, in this case it makes testing in a broader (AEM) context more difficult. It would be useful to be able to temporarily (ie until before 1.6) have an osgi config to turn this feature on or off.