[KItemModels/KLinkItemSelectionModelTest] Port QRegExp to QRegularExpression

Summary:
Use QSortFilterProxyModel::setFilterRegularExpression() instead of
setFitlerRegExp().

Test Plan: make && ctest

Reviewers: Frameworks, dfaure, broulik, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28494