Add a test for AF_UNIX socket support

[piscisaureus]

At the time of writing, insider builds support Unix sockets: https://blogs.msdn.microsoft.com/commandline/2017/12/19/af_unix-comes-to-windows
It seems that this type of socket works just fine with wepoll, however it'd be nice to add a test for this.

[fanzeyi]

Hi, I tried to just duplicate test-success-events.c and replacing AF_INET with AF_UNIX plus some setups for UDS. It seems wepoll supports UDS out of the box. The commit is at fanzeyi@fbb963a if you are interested.

(I do not really fully understand what I am doing. I have zero practical knowledge on epoll & socket on Windows. Apologies if this doesn't make any sense.)