selftests: netdevsim: add ethtool features to macsec offload tests

The test verifies that available features aren't changed by toggling
offload on the device. Creating a device with offload off and then
enabling it later should result in the same features as creating the
device with offload enabled directly.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/ba801bd0a75b02de2dddbfc77f9efceb8b3d8a2e.1730929545.git.sd@queasysnail.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>