Fix test failing on systems without IPv6 support

[jackwilsdon]

This test fails if IPv6 is disabled:

____________________ TestBaseServerRequests.test_bind_fail _____________________

self = <radicale.tests.test_server.TestBaseServerRequests object at 0x7ffff4e2b8f0>

    def test_bind_fail(self) -> None:
        for address_family, address in [(socket.AF_INET, "::1"),
                                        (socket.AF_INET6, "127.0.0.1")]:
>           with socket.socket(address_family, socket.SOCK_STREAM) as sock:

radicale/tests/test_server.py:144: 

This PR just copies the IPv6 check from test_ipv6, skipping an address if it's not supported.