selftests: forwarding: introduce helper for standard ethtool counters

Counters for the MAC Merge layer and preemptible MAC have standardized
so far on using structured ethtool stats as opposed to the driver
specific names and meanings.

Benefit from that rare opportunity and introduce a helper to lib.sh for
querying standardized counters, in the hope that these will take off for
other uses as well.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>