tests: Skip GSocket tests if setting up a server fails

There are various reasons why setting up a server might fail; it
reliably fails on AWS with IPv6 addresses (are we binding to the right
address?). Since we’re trying to test GSocket as a client, skip tests
where that happens.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=795234