Updated corelib's unit tests to use QSignalSpy's functor constructor

The intent is to provide compile time validation of signals and to help
detect signal overloading in the future.

Change-Id: I9d5d46ed4b70c5d0cd407deb5928b1e76d37e007
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>