add a simple smoke test for slaves by using kioclient5

Summary:
this is meant to give a fairly generic way to easily test core
functionality of any io slave

Test Plan: ran against sftp and smb and it's all lovely

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: feverfew, kde-frameworks-devel, kfm-devel

Tags: Dolphin, Frameworks

Differential Revision: https://phabricator.kde.org/D24076