gio/tests/network-address: fix when no network is available

GResolver doesn't do full validation of its inputs, so in some of
these tests, the fact that we were getting back
G_RESOLVER_ERROR_NOT_FOUND is because the junk string was getting
passed to an upstream DNS resolver, which returned NXDOMAIN. But if
there's no network on the machine then we'd get
G_RESOLVER_ERROR_INTERNAL instead in that case.