Add a number of tests for layer squashing

LayoutTests/compositing/squashing/squash-onto-nephew-expected.txt

 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (children 1
         (GraphicsLayer
-          (position 50.00 50.00)
+          (position 25.00 25.00)
           (bounds 300.00 300.00)
-          (drawsContent 1)
+          (contentsOpaque 1)
           (children 1
             (GraphicsLayer
               (bounds 300.00 300.00)
               (children 1
                 (GraphicsLayer
-                  (bounds 285.00 300.00)
-                  (children 1
+                  (children 2
                     (GraphicsLayer
-                      (bounds 285.00 1000.00)
+                      (position 100.00 100.00)
+                      (bounds 100.00 100.00)
+                      (contentsOpaque 1)
+                    )
+                    (GraphicsLayer
+                      (position 15.00 15.00)
+                      (bounds 180.00 190.00)
+                      (drawsContent 1)
                     )
                   )
                 )
               )
             )
           )
         )
       )
     )
   )
 )