bug 426374: Notify IServiceListeners of previously registered services

- Register IServiceListener _after_ explicit discovery to reduce
likelyhood of seeing the same discovery event twice
- Lock TestListener early to not miss discovery events
- Wrap discovery events send nested discovery containers in
CompositeContainer before listeners are notified
- Let TestListener only react to its accompanied unit test (to prevent
test failures due to dangling discovery events) 

Bug-URL: https://bugs.eclipse.org/bugs/show_bug.cgi?id=426374 
9 files changed