python: Fix tests on MinGW

Add the directory containing libxml2.dll with os.add_dll_directory to
make tests work on MinGW.

This has changed in Python 3.8 but for some reason, the issue only
turned up with Python 3.11 on MinGW. Contrary to documentation, copying
libxml2.dll into the directory containing the .pyd file doesn't work.

python/tests/error.py

@@ -4,6 +4,7 @@
 # functions defined in Python.
 #
 import sys
+import setup_test
 import libxml2
 
 # Memory debug specific