selftests: forwarding: Add a helper to skip test when using veth pairs

A handful of tests require physical loopbacks to be used instead of veth
pairs. Add a helper that these tests will invoke in order to be skipped
when executed with veth pairs.

Fixes: 64916b57

 ("selftests: forwarding: Add speed and auto-negotiation test")
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/20230808141503.4060661-7-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>