Make copy of test_mailbox.py. We'll still want to check the backward

Make copy of test_mailbox.py.  We'll still want to check the backward
compatibility classes in the new mailbox.py that I'll be committing in
a few minutes.

One change has been made: the tests use len(mbox) instead of len(mbox.boxes).
The 'boxes' attribute was never documented and contains some internal state
that seems unlikely to have been useful.