selftests/net: Rectify key counters checks

selftests/net: Rectify key counters checks

As the names of (struct test_key) members didn't reflect whether the key
was used for TX or RX, the verification for the counters was done
incorrectly for asymmetrical selftests.

Rename these with _tx appendix and fix checks in verify_counters().
While at it, as the checks are now correct, introduce skip_counters_checks,
which is intended for tests where it's expected that a key that was set
with setsockopt(sk, IPPROTO_TCP, TCP_AO_INFO, ...) might had no chance
of getting used on the wire.

Fixes the following failures, exposed by the previous commit:
> not ok 51 server: Check current != rnext keys set before connect(): Counter pkt_good was expected to increase 0 => 0 for key 132:5
> not ok 52 server: Check current != rnext keys set before connect(): Counter pkt_good was not expected to increase 0 => 21 for key 137:10
>
> not ok 63 server: Check current flapping back on peer's RnextKey request: Counter pkt_good was expected to increase 0 => 0 for key 132:5
> not ok 64 server: Check current flapping back on peer's RnextKey request: Counter pkt_good was not expected to increase 0 => 40 for key 137:10

Cc: Mohammad Nassiri <mnassiri@ciena.com>
Fixes: 3c3ead555648 ("selftests/net: Add TCP-AO key-management test")
Signed-off-by: Dmitry Safonov <dima@arista.com>
Link: https://lore.kernel.org/r/20240130-tcp-ao-test-key-mgmt-v2-2-d190430a6c60@arista.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>