Unit tests may not work with SplitView

[JeffryBooher]

The unit tests for this extension uses an API that has been removed EditorManager.setEditorHolder() You can use the MainViewManager._init() method to initialize to a DOM element.

See:
https://github.com/adobe/brackets/wiki/SplitView-Extension-Migration-Guide

branch:
Jeff/splitview-factory

Let us know if you need help,
Thanks!