GD-518: Fix run/debug of CS tests via the context menu fails

[MikeSchulze]

Why

see https://github.com/MikeSchulze/gdUnit4/issues/518

What

• Use of localize_path for all resource paths used (cs scripts contain the system path)
• create_tree_item() has been corrected to split the test path into tree items depending on the view mode.
• It takes into account if the test root folder does not exist to split the full path then