selftests: lib: Define more kselftest exit codes

The following patches will operate with more exit codes besides
ksft_skip. Add them here.

Additionally, move a duplicated skip exit code definition from
forwarding/tc_tunnel_key.sh. Keep a similar duplicate in
forwarding/devlink_lib.sh, because even though lib.sh will have
been sourced in all cases where devlink_lib is, the inclusion is not
visible in the file itself, and relying on it would be confusing.

Cc: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/545a03046c7aca0628a51a389a9b81949ab288ce.1711464583.git.petrm@nvidia.com


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