providers/aws: vpc dns settings tests

cc/ @deoxxa see diff for how to add tests – quite straight forward.

Normally, I would like to add a check to ensure that the attribute
was updated remotely (by seeing what it's value with AWS is) but
I don't think we have that value returned.
helper/schema: diffing should use ResourceData for guidance
helper/schema: track map of swaps
helper/schema: remove swap tracking
helper/schema: can read and get the state of sets
helper/schema: verify that sets remove duplicates
helper/schema: can set sets
helper/schema: Set operations
helper/schema: set diff tests