igb: update testing done by ethtool

Most of the code for the testing has pretty much become stale at this point
and is need of update.  This update just streamlines most of the code,
widens the range of interrupt testing.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>