selftests: bonding: Add net/forwarding/lib.sh to TEST_INCLUDES

In order to avoid duplicated files when both the bonding and forwarding
tests are exported together, add net/forwarding/lib.sh to TEST_INCLUDES and
include it via its relative path.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Tested-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Reviewed-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>