selftests/net: Provide tcp-ao counters comparison helper

selftests/net: Provide tcp-ao counters comparison helper

Rename __test_tcp_ao_counters_cmp() into test_assert_counters_ao() and
test_tcp_ao_key_counters_cmp() into test_assert_counters_key() as they
are asserts, rather than just compare functions.

Provide test_cmp_counters() helper, that's going to be used to compare
ao_info and netns counters as a stop condition for polling the sockets.

Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://patch.msgid.link/20250319-tcp-ao-selftests-polling-v2-2-da48040153d1@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>