selftests/tls: add a bidirectional test

Add a simple test which installs the TLS state for both directions,
sends and receives data on both sockets.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>