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.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -164,6 +164,17 @@ check_port_mab_support()
 	fi
 }
 
+skip_on_veth()
+{
+	local kind=$(ip -j -d link show dev ${NETIFS[p1]} |
+		jq -r '.[].linkinfo.info_kind')
+
+	if [[ $kind == veth ]]; then
+		echo "SKIP: Test cannot be run with veth pairs"
+		exit $ksft_skip
+	fi
+}
+
 if [[ "$(id -u)" -ne 0 ]]; then
 	echo "SKIP: need root privileges"
 	exit $ksft_skip