python: Fix tests on MinGW

Add the directories containing required DLLs with os.add_dll_directory
to make tests work on MinGW.

python/tests/basic.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python
 import os
 import sys
+import setup_test
 import libxml2
 # Memory debug specific
 libxml2.debugMemory(1)