Move shadow layout to item

It turns out layouts can be large[citation needed].  The layout used
for a prior buildset is only of historic interest.  The shadow layout
is effectively used only to freeze a job graph, and once a buildset
has been superceded, that is no longer necessary.  Drop the reference
at this point to allow the memory to be reclaimed sooner.

Change-Id: I2bf651ab99f1b1bfbe0e2442cf26c40475022fe5
diff --git a/tests/unit/test_scheduler.py b/tests/unit/test_scheduler.py
index 9e5e36c..267bedd 100755
--- a/tests/unit/test_scheduler.py
+++ b/tests/unit/test_scheduler.py
@@ -5589,7 +5589,7 @@
                 queue = queue_candidate
                 break
         queue_item = queue.queue[0]
-        item_dynamic_layout = queue_item.current_build_set.layout
+        item_dynamic_layout = queue_item.layout
         dynamic_test_semaphore = \
             item_dynamic_layout.semaphores.get('test-semaphore')
         self.assertEqual(dynamic_test_semaphore.max, 1)