selftests: tls: add tests for zero-length records

jira LE-4311
cve CVE-2025-39682
Rebuild_History Non-Buildable kernel-5.14.0-570.49.1.el9_6
commit-author Jakub Kicinski <kuba@kernel.org>
commit a61a3e9
Empty-Commit: Cherry-Pick Conflicts during history rebuild.
Will be included in final tarball splat. Ref for failed cherry-pick at:
ciq/ciq_backports/kernel-5.14.0-570.49.1.el9_6/a61a3e96.failed

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>
(cherry picked from commit a61a3e9)
	Signed-off-by: Jonathan Maple <jmaple@ciq.com>

# Conflicts:
#	tools/testing/selftests/net/tls.c

Author: PlaidCat