LightEditorTest : Fix testing w/ multiple columns

- Getting any old `_GafferSceneUI._LightEditorInspectorColumn` will also
include the new `_GafferSceneUI._LightEditorSoloColumn`, which we don't
want.
- It turns out creating a Python list with multiple entries using the
`[] * n` syntax creates shallow copies of the list contents. Therefore
when modifying the selection, it was affecting all columns because the
list members were all the same `IECore.PathMatcher()`. This caused
incorrect results when both the "Mute" and "Solo" column were toggled.

python/GafferSceneUITest/LightEditorTest.py

@@ -637,12 +637,12 @@ def testLightMuteAttribute( toggleCount, toggleLocation, newStates ) :
 
 			if muteIndex is None :
 				for i, c in zip( range( 0, len( columns ) ), columns ) :
-					if isinstance( c, _GafferSceneUI._LightEditorInspectorColumn ) :
+					if isinstance( c, _GafferSceneUI._LightEditorMuteColumn ) :
 						muteIndex = i
 
 			self.assertIsNotNone( muteIndex )
 
-			selection = [IECore.PathMatcher()] * len( columns )
+			selection = [ IECore.PathMatcher() for i in range( 0, len( columns ) ) ]
 			for path in togglePaths :
 				selection[muteIndex].addPath( path )