igb: add single vector msi-x testing to interrupt test

This change adds testing of the first msix vector to the interrupt testing.
This should help with determining the cause of interrupt issues when they are
encountered.

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>