selftests: tls: test TCP stealing data from under the TLS socket

Check a race where data disappears from the TCP socket after
TLS signaled that its ready to receive.

  ok 6 global.data_steal
  #  RUN           tls_basic.base_base ...
  #            OK  tls_basic.base_base

Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250807232907.600366-2-kuba@kernel.org


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