selftests: Migrate nsim-only MACsec tests to Python

selftests: Migrate nsim-only MACsec tests to Python

Move MACsec offload API and ethtool feature tests from
tools/testing/selftests/drivers/net/netdevsim/macsec-offload.sh to
tools/testing/selftests/drivers/net/macsec.py using the NetDrvEnv
framework so tests can run against both netdevsim (default) and real
hardware (NETIF=ethX). As some real hardware requires MACsec to use
encryption, add that to the tests.

Netdevsim-specific limit checks (max SecY, max RX SC) were moved into
separate test cases to avoid failures on real hardware.

Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20260408115240.1636047-2-cratiu@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>