QDir 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 a hidden friend.
Use QTestPrivate::testEqualityOperators() helper function in unit-tests.
Task-number: QTBUG-120303
Change-Id: I86c2ba18b8b114efd9f62fc2fd628bc9065b04b2
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>