converted the existing testPlanState() from terraform.State to
states.State to fix various plan tests.
reverted the "bandaid" in plans/planfile/tfplan.go - at this moment the
backend tests do not include backend configuration, and so the planfile
package can write the plan file but not read it back in. That will be
revisted in a separate track of work.
We are changing the behaviour of the "stub" diff operation to just have
the pre/post-diff hooks skipped on eval, meaning that the test against
CountHook will ultimately be meaningless and fail, hence we need a
different test here that tests it on a more general level.
Kristin Laemmert