Split XDGTest to allow testing both V5 and V6

Summary:
We want to test V6 using the exact same tests.

We already use the QtTest rows inside the XdgClient, using that also for
this separation gets quite complicated.

Test Plan: Current test still passes.

Reviewers: Plasma

Subscribers: plasma-devel, Frameworks

Tags: Plasma on Wayland, Frameworks

Differential Revision: https://phabricator.kde.org/D5801