[KDeclarative autotests] Replace fork of old modeltest with QAbstractItemModelTester

Test Plan: make && ctest

Reviewers: mart, davidedmundson, vkrause

Reviewed By: vkrause

Subscribers: kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T11553

Differential Revision: https://phabricator.kde.org/D23895