ktls_tests: Ignore spurious errors from shutdown(2).

For some of the "bad size" tests, the remote end can notice the error
and drop the connection before the test program returns from write to
call shutdown. In that case, shutdown fails with ENOTCONN. Permit
these ENOTCONN errors without failing the test.

Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D37693