Enable Python hash seed randomization in tests

Unit tests should run with hash seed randomization
on, to ensure code is not introduced that relies on
ordered access of dicts, sets, etc.

Python 3.3 enables this at runtime by default.

Change-Id: I792cf9d24867b0bb8c9c43030ed80d5fd6ce6ec0

tox.ini

@@ -5,7 +5,6 @@ skip_missing_interpreters = True
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
-         PYTHONHASHSEED=0
 install_command =
   pip install {opts} {packages}
 commands = find . -type f -name "*.py[c|o]" -delete