Make the test `dupe_port` Windows-only
Make the test `dupe_port` Windows-only

Because the socket is configured with the SO_REUSEADDR flag on
non-Windows systems, you can successfully launch multiple instance of a
civet webserver on the same host-port pair. The test is now only run on
windows platform because the socket is configured with the
SO_EXCLUSIVEADDRUSE flag there.

wycats/rust-civet