driver code, so that each test gets this; it had been done inconsistently.
Remove the lines that set the variables holding dom objects to None; not
needed since the interpreter cleans up locals on function return.
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.)
