Backport r58453:

Backport r58453:
Let the O/S supply a port if none of the default ports can be used.
This should make the tests more robust at the expense of allowing
tests to be sloppier by not requiring them to cleanup after themselves.
(It will legitamitely help when running two test suites simultaneously
or if another process is already using one of the predefined ports.)

This will hopefully fix test_asynchat.