Update Lib/test/test_minidom.py to use the unittest module.

Also:  I noticed the test_minidom.py contains some code (now long dead) to check for reference cycles.  The code is active only when Node.allnodes exists; this hasn't existed for over four years now.

After re-implementing that test using gc.DEBUG_SAVEALL, I found that many tests failed because of missing unlink() calls.  I added them.

After that, one test was still failing.  This is because expatbuilder.parseString() and friends do not clean up some cyclic references on error.  The patch includes a fix for that (in Lib/xml/dom/expatbuilder.py).  An alternative would be to ditch the cycle-checking test entirely.  I can submit a patch that does that instead, if desired.