SceneTestCase : Use prefix in set comparisons in `assertScenesEqual()`
SceneTestCase : Use prefix in set comparisons in `assertScenesEqual()`

Also simplify the code for comparing pruned sets.

andrewkaufman/gaffer,GafferHQ/gaffer,hradec/gaffer,hradec/gaffer,ImageEngine/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,hradec/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,hradec/gaffer,ImageEngine/gaffer,hradec/gaffer,GafferHQ/gaffer