[Test] Ensure that the local Python module is the one being imported

Explicitly set 'interfaces/python' as the first entry in sys.path, rather
than reyling on the PYTHONPATH environment variable, which is sometimes
superseded, e.g. by virtualenv.

test/python/runTests.py

@@ -8,6 +8,8 @@ import sys
 import os
 import unittest
 
+sys.path.insert(0, os.path.abspath('../../interfaces/python'))
+
 import Cantera
 
 Cantera.addDirectory(os.path.join(os.path.split(os.getcwd())[0], 'data'))