selftests/net/tls: add test for MSG_EOR

As the recent patch is modifying the behaviour for TLS re MSG_EOR
handling we should be having a test for it.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20230726191556.41714-4-hare@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>