Handle compare in test_pre_live_migration_volume_backed* directly

We can't use assertJsonEqual here even if that function deserializes
the nested connection_info_json. When deserializing the "bdms" entry
in the migrate_data object, it creates a random order on the list
which can still cause mismatch errors.

So rather than rely on assertJsonEqual handling this, that change
is reverted and we simply deserialize the connection_info_json
in the test in question before comparing to our expected data.

This reverts commit ac9acce283a28326c6f9af4a158e6f3c1e09db45.

Change-Id: I7bba91e490a9ac5d0bbffa90e9cbefe60bb31f27
Closes-Bug: #1779711

nova/test.py

@@ -489,18 +489,7 @@ class TestCase(testtools.TestCase):
                 return sorted(items)
             return x
 
-        def try_deserialize(entry):
-            """Try to deserialize string entry; if not json, return entry"""
-            if isinstance(entry, six.string_types):
-                try:
-                    return jsonutils.loads(entry)
-                except ValueError:
-                    pass
-            return entry
-
         def inner(expected, observed, path='root'):
-            expected = try_deserialize(expected)
-            observed = try_deserialize(observed)
             if isinstance(expected, dict) and isinstance(observed, dict):
                 self.assertEqual(
                     len(expected), len(observed),