selftests: tls: check that disconnect does nothing

selftests: tls: check that disconnect does nothing

"Inspired" by syzbot test, pre-queue some data, disconnect()
and try to receive(). This used to trigger a warning in TLS's strp.
Now we expect the disconnect() to have almost no effect.

Link: https://lore.kernel.org/67e6be74.050a0220.2f068f.007e.GAE@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20250404180334.3224206-2-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>