tests/unix_seqpacket: remove EMSGSIZE tests
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.
Reviewed by: tuexen, asomers
Differential Revision: https://reviews.freebsd.org/D43756