Tests for LayerTreeManager.


== Running test suite: LayerTreeManager
-- Running test case: LayerTreeManager.layersForNode.root
PASS: Returned array should include all layers.
PASS: Array elements should be Layer instances.

-- Running test case: LayerTreeManager.layerTreeMutations.overlappingNodeIds
PASS: Preserved layers should be correct.
PASS: Added layers should be correct.
PASS: Removed layers should be correct.