selftests: forwarding: ethtool_extended_state: Skip when using veth pairs

Ethtool extended state cannot be tested with veth pairs, resulting in
failures:

 # ./ethtool_extended_state.sh
 TEST: Autoneg, No partner detected                                  [FAIL]
         Expected "Autoneg", got "Link detected: no"
 [...]

Fix by skipping the test when used with veth pairs.

Fixes: 7d10bcce ("selftests: forwarding: Add tests for ethtool extended state")
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: Petr Machata <petrm@nvidia.com>
Tested-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20230808141503.4060661-9-idosch@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/forwarding/ethtool_extended_state.sh

@@ -108,6 +108,8 @@ no_cable()
 	ip link set dev $swp3 down
 }
 
+skip_on_veth
+
 setup_prepare
 
 tests_run