py33: iteration order of dict is unpredictable

In Python 3.3, hash randomization is enabled by default. It causes the
iteration order of dicts and sets to be unpredictable and differ
across Python runs.

Sort the metadata.keys() in reverse order to keep the test cases as is.

Close-Bug #1233405

Change-Id: I6251f18c9bc3a7842be9efeb5131c82919020e75
Merge "py33: iteration order of dict is unpredictable"