Fix exp tests cast protodriver to naivediffdriver

A recent change made `graphdriver.NaiveDiffDriver` from a function to a
struct. New function is `graphdriver.NewNaiveDiffDriver.

The graphdriver plugin PR was created (and tests run) before this change
was introduced causing the failure to not be seen until after merge.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>