selftests: forwarding: ethtool: Skip when using veth pairs

Auto-negotiation cannot be tested with veth pairs, resulting in
failures:

 # ./ethtool.sh
 TEST: force of same speed autoneg off                               [FAIL]
         error in configuration. swp1 speed Not autoneg off
 [...]

Fix by skipping the test when used with veth pairs.

Fixes: 64916b57 ("selftests: forwarding: Add speed and auto-negotiation test")
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/20230808141503.4060661-8-idosch@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -286,6 +286,8 @@ different_speeds_autoneg_on()
 	ethtool -s $h1 autoneg on
 }
 
+skip_on_veth
+
 trap cleanup EXIT
 
 setup_prepare