Finally got around to figuring out and documenting why this test fails

Finally got around to figuring out and documenting why this test fails
on Windows.  The test_sequence() ERROR is easily repaired if we're
willing to add an os.unlink() line to mhlib's updateline().  The
test_listfolders FAIL I gave up on -- I don't remember enough about Unix
link esoterica to recall why a link count of 2 is something a well-
written program should be keenly interested in <wink>.