Add unit tests for Folder View

Summary: Add some tests for FolderModel and Positioner classes.

Reviewers: #plasma, ervin, hein, mlaurent, aacid, dvratil, franckarrecot, renatoo, olivierjg

Reviewed By: #plasma, hein, mlaurent

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8447