tests/unix_seqpacket: test that data can be sent before accept(2)

This is undocumented feature of PF_UNIX/SOCK_STREAM and thus of
PF_UNIX/SOCK_SEQPACKET, too. Put the test into this file, since this file
is most advanced and has all primitives to write this test in minimum
number of lines.

Differential Revision: https://reviews.freebsd.org/D43853