ktls: Add simple transmit tests of kernel TLS.

Note that these tests test the kernel TLS functionality directly.
Rather than using OpenSSL to perform negotiation and generate keys,
these tests generate random keys send data over a pair of TCP sockets
manually decrypting the TLS records generated by the kernel.

Reviewed by: markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D32652