unix: add SockaddrUnix tests on linux
unix: add SockaddrUnix tests on linux

Follow the existing examples of the SockaddrTIPC and SockaddrL2TPIP{,6}
tests.

Handling of abstract socket addresses is different on linux than on
other platforms, thus these are currently linux-specific.

Change-Id: I66686d979e1dd317ab7a8a6dd85f98e670ad89b6
Reviewed-on: https://go-review.googlesource.com/c/sys/+/228764
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

multiplay/golang-sys,multiplay/golang-sys,golang/sys,golang/sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys