Fix database password extraction from environment variables in the tests settings.

tests/settings.py

                                  'django.db.backends.postgresql'),
         'NAME': os.environ.get('DATABASE_NAME', 'oscar'),
         'USER': os.environ.get('DATABASE_USER', None),
-        'PASSWORD': os.environ.get('DATABASE_USER', None),
+        'PASSWORD': os.environ.get('DATABASE_PASSWORD', None),
     }
 }