QTestLib: clean up qCompare specialisation/overload mix

QTestLib: clean up qCompare specialisation/overload mix

qCompare() was both overloaded and specialised, but always as a template.
This lead to the QIcon specialisation actually invoking
   qCompare(QFlags<void*>, ...)
when specifically asking for qCompare<void*>() (detected by adding
underlying-type detection to QFlags).

Fix by preferring overloading and not specialising anything.

Change-Id: Ie001ebb9dfb0847c6c33a3f45177a61579fd61ee
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>