selftests: tls: add tests for zero-length records

Test various combinations of zero-length records.
Unfortunately, kernel cannot be coerced into producing those,
so hardcode the ciphertext messages in the test.

Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20250820021952.143068-2-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>