test-socket-util: fix test for address parsing w/o IPv6 support

test-socket-util: fix test for address parsing w/o IPv6 support

sockaddr_pretty() is the function that is ultimately called to print
the address, and afaics, it'll always use 0.0.0.0 if family==AF_INET.
So the test must have always been broken on systems with IPv6 support.

Fixes a failure in semaphore debian autopkgtest ci:
"65535" → ✓ 0 → "0.0.0.0:65535" (expect 0 / "[::]:65535") 15:15
Assertion 'streq(out, expected ?: in)' failed at src/test/test-socket-netlink.c:32, function test_socket_address_parse_one(). Aborting. 15:15