[C++] Relax timing requirements in SubscriptionTest
[C++] Relax timing requirements in SubscriptionTest

Sporadically, some tests in the SubscriptionTest fail, most likely
because of time bounds not able to be fulfilled by the build server
due to scheduling delays on the host. Thus, reduce the tests to
the essential things, avoiding to run into such timing issues.

Change-Id: Ie5d8815b8451d1d4cbd106c74999c74930c8a958

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