tests/unix_seqpacket: remove EMSGSIZE tests

[glebius]

Summary

These tests were not testing conformance to the specification, rather than
the limitation of our implementation. The specification doesn't say that
a SOCK_SEQPACKET shall ever return EMSGSIZE. It says:

The SOCK_SEQPACKET socket type is similar to the SOCK_STREAM type,
and is also connection-oriented. The only difference between these
types is that record boundaries are maintained using the
SOCK_SEQPACKET type. A record can be sent using one or more output
operations and received using one or more input operations, but a
single operation never transfers parts of more than one record.
Record boundaries are visible to the receiver via the MSG_EOR flag
in the received message flags returned by the recvmsg() function. It
is protocol-specific whether a maximum record size is imposed.

The EMSGSIZE is specified as 'message is too large to be sent all at once,
as the socket requires'. Indeed existing implementation that has
unix/seqpacket marked as PR_ATOMIC has such a limitation. But future
implementation won't have, thus remove the tests.

[asomers]

So in the future it will be possible to send a single record with multiple send syscalls? In that case we can certainly remove these tests. However, in the future it will be important to ensure that we can send messages larger than the socket buffer size, right? In that case, I think we should leave these tests here for now, and update them atomically when the new behavior is committed. That way we won't forget.

[glebius]

I will provide tests that you can write a record larger than socket buffer with a blocking write, or with several non-blocking writes. I'm going to commit them together with or after the new implementation.

A preview is here (still very immature code): https://github.com/freebsd/freebsd-src/commit/0ed33339e1a012a4ca8a583193cac8510332dba3