Allow eventfilter to match paths without system name for source

Previously the full path was needed when specifying the source, including the actor system name.
This commit makes it possible to use paths without system name (paths with system name are still allowed):

    EventFilter.Exception<ActorInitializationException>(source:"/user/failing-actor").ExpectOne(()=>
      ActorOf(Props.Create<FailsOnInstantiationActor>, "failing-actor")
    );
Merge pull request #522 from HCanber/testkit-filter-source-without-system-name

Allow EventFilter to match paths without system name for source