net: run unix socket stream tests on Windows

net: run unix socket stream tests on Windows

The net package supports Unix domain sockets on Windows, but most of
the tests related to them are skipped.

This CL unskip the SOCK_STREAM tests. SOCK_DGRAM probablye can also
make to work, but that will come in a follow-up CL.

Change-Id: If9506a8af57e9bfe58bd7b48a98fc39335627a61
Reviewed-on: https://go-review.googlesource.com/c/go/+/660915
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>