If a derived test case implements the same private slot as one of
its base classes one of these information got lost as both were
using the same key to store the information at.
Additionally take care of what is possible and sensible when using
QtTest and deriving test cases from others.

Task-number: QTCREATORBUG-17522
Change-Id: I0d2a47c820d5eb002f8bdd851a07a4774e9838f0
Reviewed-by: David Schulz <david.schulz@qt.io>