Python3: fix test_stack_list_with_args()

The expected_url in test_stack_list_with_args() does not always match the URL
actually used in the code. This patch fixes this issue by:
- using urlutils.urlencode() in both the code and the tests, so that changes in
 the urlencode() implementation do not affect the tests;
- setting the PYTHONHASHSEED environment variable to 0 when running the tests,
  so that randomisation is disabled and urlutils.urlencode() has predictable
  results.

Change-Id: I5b50bad98f5c237cdc12e4ab75f21917ca0ef30d
Merge "Python3: fix test_stack_list_with_args()"