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: 81573b18

 ("selftests/net/forwarding: add Makefile to install tests")
Reported-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Closes: https://lore.kernel.org/netdev/856d454e-f83c-20cf-e166-6dc06cbc1543@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/20230808141503.4060661-2-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>