[test] skip IPv6 socket tests without IPv6, fixes #1068

There are systems without INET6 family support at all. It is a kernel option afterall.
Thanks to basiliscos for reporting
ChangeLog: add GH #1068
Merge branch 'socketipv6-gh1068'

Fix confirmed by user

./parrot t/pmc/socket.t
1..23
ok 1 - Instantiated a Socket PMC
ok 2 - PMC has correct type
ok 3 - can get_fd a Socket
ok 4 - Socket read returns 0 when not connected
ok 5 - Cloned a Socket PMC
ok 6 - Cloned PMC has correct type TT#1820
ok 7 - get_bool on closed Socket
ok 8 - Closed a Socket
ok 9 - A closed Socket returns False
ok 10 - Socket is_closed returned 1 to new socket
ok 11 - Created a TCP Socket
ok 12 - Created a UDP Socket
ok 13 - Socket getprotobyname(icmp) returned 1
ok 14 - Socket getprotobyname(tcp) returned 6
ok 15 - empty protocol name does not coredump
ok 16 - non-existent protocol name does not coredump
ok 17 - Server process started
ok 18 - socket
ok 19 - send
ok 20 - recv
ok 21 - send() big
ok 22 - read(1024) chunked
ok 23 - Exit status of server process