Tests hit testing in Layers3DView

Initial state
State of layers:
hovered layer: none
selected layer: content root

Hovering content root
State of layers:
hovered layer: none
selected layer: content root

Selecting layer b
State of layers:
hovered layer: none
selected layer: layer b

Hovering layer a
State of layers:
hovered layer: layer a
selected layer: layer b

Selecting content root
State of layers:
hovered layer: none
selected layer: content root