helper/resource: ignore data resource diffs during destroy

When testing destroy, the test harness calls Refresh followed by Plan,
with the expectation that the resulting diff will be empty.

Data resources challenge this expectation, because they will always be
instantiated during refresh if their configuration isn't computed, and so
the subsequent diff will want to destroy what was instantiated.

To work around this, we make an exception that data resource destroy
diffs may appear in the plan but nothing else.

This fixes #6713.

helper/resource/testing_config.go

@@ -3,6 +3,7 @@ package resource
 import (
 	"fmt"
 	"log"
+	"strings"
 
 	"github.com/hashicorp/terraform/terraform"
 )
@@ -108,7 +109,32 @@ func testStep(
 	if p, err = ctx.Plan(); err != nil {
 		return state, fmt.Errorf("Error on second follow-up plan: %s", err)
 	}
-	if p.Diff != nil && !p.Diff.Empty() {
+	empty := p.Diff == nil || p.Diff.Empty()
+
+	// Data resources are tricky because they legitimately get instantiated
+	// during refresh so that they will be already populated during the
+	// plan walk. Because of this, if we have any data resources in the
+	// config we'll end up wanting to destroy them again here. This is
+	// acceptable and expected, and we'll treat it as "empty" for the
+	// sake of this testing.
+	if step.Destroy {
+		empty = true
+
+		for _, moduleDiff := range p.Diff.Modules {
+			for k, instanceDiff := range moduleDiff.Resources {
+				if !strings.HasPrefix(k, "data.") {
+					empty = false
+					break
+				}
+
+				if !instanceDiff.Destroy {
+					empty = false
+				}
+			}
+		}
+	}
+
+	if !empty {
 		if step.ExpectNonEmptyPlan {
 			log.Printf("[INFO] Got non-empty plan, as expected:\n\n%s", p)
 		} else {