selftests: forwarding: ethtool_mm: Skip when MAC Merge is not supported

MAC Merge cannot be tested with veth pairs, resulting in failures:

 # ./ethtool_mm.sh
 [...]
 TEST: Manual configuration with verification: swp1 to swp2          [FAIL]
         Verification did not succeed

Fix by skipping the test when the interfaces do not support MAC Merge.

Fixes: e6991384

 ("selftests: forwarding: add a test for MAC Merge layer")
Reported-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Closes: https://lore.kernel.org/netdev/adc5e40d-d040-a65e-eb26-edf47dac5b02@alu.unizg.hr/
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20230808141503.4060661-11-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>