of intermittent test failures.  In ConnectionTests, a random port was
selected without checking if it was in use.  testZEO.get_port (moved
to forker) picked a random port, checking if it was in use, but
clients actually used that port *and* the following one.  Now check
that the returned and subsequent ports are free. (Of course, they
could get used betweed the time they're selected and the time they are
used by the test. Oh well.