testing: Use a copy of pre-destroy state in destroy check

In the acceptance testing framework, it is neccessary to provide a copy
of the state _before_ the destroy is applied to the check in order that
it can loop over resources to verify their destruction. This patch makes
a deep copy of the state prior to applying test steps which have the
Destroy option set and then passes that to the destroy check.

helper/resource/testing.go

@@ -247,6 +247,11 @@ func testStep(
 		log.Printf("[WARN] Test: Step plan: %s", p)
 	}
 
+	// We need to keep a copy of the state prior to destroying
+	// such that destroy steps can verify their behaviour in the check
+	// function
+	stateBeforeApplication := state.DeepCopy()
+
 	// Apply!
 	state, err = ctx.Apply()
 	if err != nil {
@@ -255,10 +260,16 @@ func testStep(
 
 	// Check! Excitement!
 	if step.Check != nil {
+		if step.Destroy {
+			if err := step.Check(stateBeforeApplication); err != nil {
+				return state, fmt.Errorf("Check failed: %s", err)
+			}
+		} else {
 			if err := step.Check(state); err != nil {
 				return state, fmt.Errorf("Check failed: %s", err)
 			}
 		}
+	}
 
 	// Now, verify that Plan is now empty and we don't have a perpetual diff issue
 	// We do this with TWO plans. One without a refresh.