The class had operator==() and operator!=() defined as public member
functions, so use QT_CORE_REMOVED_SINCE and removed_api.cpp to get
rid of these methods and replace them with hidden friends.
Extend unit-tests by using the helper functions from QTestPrivate.
Task-number: QTBUG-104111
Change-Id: Ib9ca613005e2f1521dea5e3cd9e2baa0b47fede4
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>