flag is true, is set to a StringIO object that silently collects all
debug messages.  This is triggered by the Node._debug=1 statement at
the top of test_minidom.py.  After the tests, we better delete that
StringIO object to avoid wasting memory.  We also reset the _debug
flag.  (Note that this is an undetectable memory leak, and the memory
doesn't get collected by the cycle-gc either, because it's all
reachable -- it's just useless.)