test: Tweak tcp-accept-stress one last time

The tcp-accept-stress, despite the previous modifications, is still deadlocking
on the osx buildbots. When building/testing/running repeatedly locally, it was
discovered that the test would often fail with TcpStream::connect returning the
error `address not available`.

This test opens up quite a large number of sockets, and it looks like by default
osx isn't the speediest at recycling those sockets for further use.

The test has been modified (and verified) to not deadlock in this error case,
and the test is not just officially ignored on OSX (with no FIXME). I believe
that we'll get good coverage of the relevant code on the linux builders, so this
isn't so much of a loss.

At the same time I turned down the stress parameters to hopefully lighten the
socket load on other platforms.