test_net IPv6 issues

My container-based CI environment can create IPv6 sockets, but there are no IPv6 addresses assigned inside the container. (I cannot change the CI environment.) This causes test_net to assert and crash the build because test_dns_tcp() does not notice the socket failures and it attempts socket operations on a closed file descriptor. The first patch attached fixes test_net so that it propagates errors from _sync().

0001-tests-contrib-test_net-Don-t-assert-on-net_stream_re.patch

After fixing the assert, test_net still has a lot of failing tests (similar to #639 (closed)). The second patch binds to INADDR_LOOPBACK instead of in6addr_loopback which causes the tests to pass.

0002-tests-contrib-test_net-Use-IPv4-loopback-socket-inst.patch