AbstractClient: Fix the current VD being always added to the plasma interface

When setting-up the plasma window interface we need to call setOnAllDesktops() after the actual virtual desktops have been set.

Otherwise, the current desktop would be always added to the interface (as if the user were unsetting the "onAllDesktops" flag) and the plasma interface and KWin can get un-synced, specially if there are specific window rules.

BUG: 452171 FIXED-IN: 5.24.5

First commit improves and adapts the autotests to the use the new Wayland desktops API instead of relaying on the X11 fallback methods, and tests also that the VDs are applied correctly to the plasma interface