Use QStandardPaths::findExecutable instead of deprecated KStandardDirs::findExe

Summary:
Tests pass except for konqviewmgrtest, konqhtmltest and konqviewtes, but
they fail on master too.

Reviewers: dfaure, stefanocrocco, vkrause

Reviewed By: vkrause

Maniphest Tasks: T11559

Differential Revision: https://phabricator.kde.org/D24787