helper/resource: verify refresh+plan after each step

I forgot to add `Computed: true` when I made the "key_name" field
optional in #1751.

This made the behavior:

 * Name generated in Create and set as ID
 * Follow up plan (without refresh)
 * Name got cleared out on Read, causing a bad diff.

We can automatically catch bugs like this if we add yet another
verification step to our resource acceptance tests -> a post
Refresh+Plan that we verify is empty.

I left the non-refresh Plan verification in, because it's important that
_both_ of these are empty after an Apply.

helper/resource/testing.go

@@ -240,7 +240,8 @@ func testStep(
 		}
 	}
 
-	// Verify that Plan is now empty and we don't have a perpetual diff issue
+	// 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.
 	if p, err := ctx.Plan(); err != nil {
 		return state, fmt.Errorf("Error on follow-up plan: %s", err)
 	} else {
@@ -250,6 +251,21 @@ func testStep(
 		}
 	}
 
+	// And another after a Refresh.
+	state, err = ctx.Refresh()
+	if err != nil {
+		return state, fmt.Errorf(
+			"Error on follow-up refresh: %s", err)
+	}
+	if p, err := ctx.Plan(); err != nil {
+		return state, fmt.Errorf("Error on second follow-up plan: %s", err)
+	} else {
+		if p.Diff != nil && !p.Diff.Empty() {
+			return state, fmt.Errorf(
+				"After applying this step and refreshing, the plan was not empty:\n\n%s", p)
+		}
+	}
+
 	return state, err
 }