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>