IECoreRenderMan.RendererTest : del lightAttributes

We can't hold onto the `lightAttributes` longer than the renderer or it
will attempt to delete a shader from a null Riley pointer when cleaning
up the `lightAttributes`.

python/IECoreRenderManTest/RendererTest.py

@@ -184,6 +184,7 @@ def testMissingLightShader( self ) :
 		self.assertEqual( messageHandler.messages[0].level, IECore.MessageHandler.Level.Warning )
 		self.assertEqual( messageHandler.messages[0].message, "Unable to find shader \"BadShader\"." )
 
+		del lightAttributes
 		del light
 		del renderer