[C++] made MessageRouterTest more stable
[C++] made MessageRouterTest more stable

explicitly synchronize via a semaphore to avoid quitting
the test before the expectation is met

Change-Id: I61ed54cce8380d41f26250ea5d89bcfcd6001b1e

bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr,clive-jevons/joynr,clive-jevons/joynr,clive-jevons/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr,clive-jevons/joynr