ktls: Add a regression test to exercise socket error handling

Prior to commit 916c61a5ed37 ("Fix handling of errors from
pru_send(PRUS_NOTREADY)") this test triggered a kernel panic due to an
mbuf double free.

Reviewed by: jhb
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33517