s/image/img/g in policy_eval_test.go
s/image/img/g in policy_eval_test.go

This does not change behavior.  The next commit will import
containers/image into this file, so we need to rename the variables.

This is split only to make reviewing easier.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,masters-of-cats/image,containers/image