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
Sponsored by: The FreeBSD Foundation

(cherry picked from commit ee5686c614ba7e11e8586b2e5b02dfc550679e96)