selftests: forwarding: Skip test when no interfaces are specified

As explained in [1], the forwarding selftests are meant to be run with
either physical loopbacks or veth pairs. The interfaces are expected to
be specified in a user-provided forwarding.config file or as command
line arguments. By default, this file is not present and the tests fail:

# make -C tools/testing/selftests TARGETS=net/forwarding run_tests
[...]
TAP version 13
1..102
# timeout set to 45
# selftests: net/forwarding: bridge_igmp.sh
# Command line is not complete. Try option "help"
# Failed to create netif
not ok 1 selftests: net/forwarding: bridge_igmp.sh # exit=1
[...]

Fix by skipping a test if interfaces are not provided either via the
configuration file or command line arguments.

# make -C tools/testing/selftests TARGETS=net/forwarding run_tests
[...]
TAP version 13
1..102
# timeout set to 45
# selftests: net/forwarding: bridge_igmp.sh
# SKIP: Cannot create interface. Name not specified
ok 1 selftests: net/forwarding: bridge_igmp.sh # SKIP

[1] tools/testing/selftests/net/forwarding/README

Fixes: 81573b18f26d ("selftests/net/forwarding: add Makefile to install tests")
Reported-by: Mirsad Todorovac <[e-mail]>
Closes: https://lore.kernel.org/netdev/[e-mail]/
Signed-off-by: Ido Schimmel <[e-mail]>
Reviewed-by: Petr Machata <[e-mail]>
Tested-by: Mirsad Todorovac <[e-mail]>
Reviewed-by: Hangbin Liu <[e-mail]>
Acked-by: Nikolay Aleksandrov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

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

···
 	for ((i = 1; i <= NUM_NETIFS; ++i)); do
 		local j=$((i+1))
 
+		if [ -z ${NETIFS[p$i]} ]; then
+			echo "SKIP: Cannot create interface. Name not specified"
+			exit $ksft_skip
+		fi
+
 		ip link show dev ${NETIFS[p$i]} &> /dev/null
 		if [[ $? -ne 0 ]]; then
 			ip link add ${NETIFS[p$i]} type veth \