selftests: net: af_unix: Test connect() with different netns.

This patch add a test that checks connect()ivity between two sockets:

    unnamed socket -> bound socket
                      * SOCK_STREAM or SOCK_DGRAM
                      * pathname or abstract
                      * same or different netns

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>