test_porcelain: fix test that is flaky with hash randomization enabled

Change-Id: I297971a9b686374c5ed0c114703d923cb4e685ea
Makefile: run tests with PYTHONHASHSEED=random

This will make it harder to check in tests that have dependencies on
Python's default hash ordering.

Change-Id: Id9ddba57b2f3a489d28605443510bbebc198c921