Fix object leak in nova.tests.objects.test_fields.TestObject

Call superclass's setUp() before defining new objects so that the object
registry can be saved, and later restored, without the test objects in
it.  This prevents the objects leaking into other tests by way of the
object registry.

Change-Id: If71a055c28c1875e67d826c5c4affda7377d179d
Merge "Fix object leak in nova.tests.objects.test_fields.TestObject"