Corrections for layers unittest based on design change

Example, imagine an object Cube in collections 1 and 2 where both
collections are nested to A. Now we set a "color" property as follow:

```
Scene -> GREEN
--
A     -> RED
↳ 1   -> BLUE
↳ 2   -> -
```

In this case the object will be RED, because of A↳ 2.

Now if we have:

```
Scene -> GREEN
--
A     -> RED
↳ 1   -> -
↳ 2   -> PINK
1     -> -
--

The object will be PINK because of A↳ 2.

Note that the (top level) collection 1 doesn't influence the object color
because there are no overrides on it. The scene render settings (GREEN
in this case) are only used as fallback if an override is not set at
all.

tests/python/render_layer/test_evaluation_render_settings_i.py

@@ -22,6 +22,7 @@ class UnitTesting(RenderLayerTesting):
         self.assertEqual(clay.get('object', 'matcap_icon'), '01')
 
         clay.set('mom', 'matcap_icon', '02')
+        self.assertEqual(clay.get('extra', 'matcap_icon'), '01')
         self.assertEqual(clay.get('object', 'matcap_icon'), '02')