tests/unix_stream: test that send(2) of zero bytes is successful

Put this simple test into an existing file. We don't have a designated
file for all unix/stream tests. There is extensive unix_seqpacket_test,
but (at least right now) unix/seqpacket is not a superset of unix/stream,
but a different implementation. We have one file that does one test for
unix/stream - unix_socketpair_test. So rename it to unix_stream and start
collecting all unix/stream tests in it.