Change testlib signal dumper hooks to use signal index range

Change testlib signal dumper hooks to use signal index range

Another step towards getting rid of the class method offset
computation in QMetaObject::activate().

Since QMetaObjectPrivate::signal() is private API, this also
required adding a testlib dependency on core-private (and
getting rid of the duplicated QSignalSpyCallbackSet struct).

Change-Id: I0d830f35392a6b44fc321c5285877ec0bf437100
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>