ktls tests: Relax error checking for shutdown(2) a bit

In my test suite runs I occasionally see shutdown(2) fail with
ECONNRESET rather than ENOTCONN. soshutdown(2) will return ENOTCONN if
the socket has been disconnected (synchronized by the socket lock), and
tcp_usr_shutdown() will return ECONNRESET if the inpcb has been dropped
(synchronized by the inpcb lock). I think it's possible to pass the
first check in soshutdown() but fail the second check in
tcp_usr_shutdown(), so modify the KTLS tests to permit this.

Reviewed by: jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42277

(cherry picked from commit b08a9b86f581edf09c5a2729d877a0204499685b)