ktls: Add simple receive tests of kernel TLS.

Similar to the simple transmit tests added in
a10482ea7476d68d1ab028145ae6d97cef747b49, these tests test the kernel
TLS functionality directly by manually encrypting TLS records using
randomly generated keys and writing them to a socket to be processed
by the kernel.

Reviewed by: markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D32980

(cherry picked from commit 3e7f8a8da254cfa97af90c3aae1cb827da55fc9d)