net: remove hard-coded timeout in dialClosedPort test helper

The helper function claims that dialing a closed port should be
"nearly instantaneous", but that is empirically not the case on
OpenBSD or Windows. The tests do not appear to be particularly
sensitive to the exact upper bound otherwise, so let's just
remove the arbitrary latency assumption.

Fixes #46884

Change-Id: If00c9fdc3063da6aaf60d365d4a2ee2c94dc6df1
Reviewed-on: https://go-review.googlesource.com/c/go/+/330250


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>