Fix the regression test for PR 181741.

With r337328, the test hangs becase the sendmsg() call will block until
the receive buffer is at least partially drained. Fix the problem by
using a non-blocking socket and allowing short writes. Also assert
that a SCM_CREDS message was received if one was expected.

PR: 181741
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D16516