tests: run with PYTHONHASHSEED=random

Python set and dict iteration order is in principle undefined but usually
'quite stable'. Setting PYTHONHASHSEED=random will make the iteration order
more random in Python 2.6.8 and 2.7.3 and where it has been backported. This
can thus help spot dependencies on undefined behaviour and prevent future
problems.

tests/run-tests.py

@@ -1252,6 +1252,7 b' def main():'
     os.environ['no_proxy'] = ''
     os.environ['NO_PROXY'] = ''
     os.environ['TERM'] = 'xterm'
+    os.environ['PYTHONHASHSEED'] = 'random'
 
     # unset env related to hooks
     for k in os.environ.keys():