selftests: tls: add test for short splice due to full skmsg

We don't have a test triggering a partial splice caused by a full
skmsg. Add one, based on a program by Jann Horn.

Use MAX_FRAGS=48 to make sure the skmsg will be full for any allowed
value of CONFIG_MAX_SKB_FRAGS (17..45).

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/1d129a15f526ea3602f3a2b368aa0b6f7e0d35d5.1760432043.git.sd@queasysnail.net


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