tests/unix_seqpacket: test send(2) to a closed or aborted peer socket

In both cases the kernel returns EPIPE and delivers SIGPIPE, unless
blocked or disabled. The test isn't specific to SOCK_SEQPACKET, it is the
same for SOCK_STREAM. Put the test into this file, since it has all
primitives to write this test tersely.

Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D44146