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>

selftests: bonding: Change script interpreter

The tests changed by this patch, as well as the scripts they source, use
features which are not part of POSIX sh (ex. 'source' and 'local'). As a
result, these tests fail when /bin/sh is dash such as on Debian. Change the
interpreter to bash so that these tests can run successfully.

Fixes: d43eff0b85ae ("selftests: bonding: up/down delay w/ slave link flapping")
Tested-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

selftests: bonding: up/down delay w/ slave link flapping

Verify when a bond is configured with {up,down}delay and the link state
of slave members flaps if there are no remaining members up the bond
should immediately select a member to bring up. (from bonding.txt
section 13.1 paragraph 4)

Suggested-by: Liang Li <liali@redhat.com>
Signed-off-by: Jonathan Toppins <jtoppins@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>