vsock_test: add SOCK_SEQPACKET tests

Implement two tests of SOCK_SEQPACKET socket: first sends data by
several 'write()'s and checks that number of 'read()' were same.
Second test checks MSG_TRUNC flag. Cases for connect(), bind(),
etc. are not tested, because it is same as for stream socket.

Signed-off-by: Arseny Krasnov <arseny.krasnov@kaspersky.com>
Signed-off-by: David S. Miller <davem@davemloft.net>