selftests: tcp_ao: add a config file

Still a bit unclear whether each directory should have its own
config file, but assuming they should lets add one for tcp_ao.

The following tests still fail with this config in place:
 - rst_ipv4,
 - rst_ipv6,
 - bench-lookups_ipv6.
other 21 pass.

Fixes: d11301f6

 ("selftests/net: Add TCP-AO ICMPs accept test")
Reviewed-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://lore.kernel.org/r/20240124192550.1865743-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>