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: d43eff0b

 ("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>