testing: selftests: drivers: net: netdevsim: devlink: add test case for hard drop statistics

Add hard drop counter check testcase, to make sure netdevsim driver
properly handles the devlink hard drop counters get/set callbacks.

Signed-off-by: Oleksandr Mazur <oleksandr.mazur@plvision.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>