core: Fix TestContext2Apply_moduleDestroyOrder

This test was occasionally failing due to a missing graph edge causing it
to be non-deterministic.

The graph edge was missing because our standard schema doesn't quite match
the config fixture and so the reference checker was not finding the "blah"
argument on aws_instance.a.

This change also includes an 100ms pause for the b node just to make this
potential race more likely to hit the "wrong" ordering when the graph is
not complete.

terraform/context_apply_test.go

@@ -2583,6 +2583,13 @@ func TestContext2Apply_moduleDestroyOrder(t *testing.T) {
 		info *InstanceInfo,
 		is *InstanceState,
 		id *InstanceDiff) (*InstanceState, error) {
+
+		if is.ID == "b" {
+			// Pause briefly to make any race conditions more visible, since
+			// missing edges here can cause undeterministic ordering.
+			time.Sleep(100 * time.Millisecond)
+		}
+
 		orderLock.Lock()
 		defer orderLock.Unlock()
 
@@ -2590,6 +2597,18 @@ func TestContext2Apply_moduleDestroyOrder(t *testing.T) {
 		return nil, nil
 	}
 
+	p.GetSchemaReturn = &ProviderSchema{
+		ResourceTypes: map[string]*configschema.Block{
+			"aws_instance": {
+				Attributes: map[string]*configschema.Attribute{
+					"id":    {Type: cty.String, Required: true},
+					"blah":  {Type: cty.String, Optional: true},
+					"value": {Type: cty.String, Optional: true},
+				},
+			},
+		},
+	}
+
 	state := mustShimLegacyState(&State{
 		Modules: []*ModuleState{
 			&ModuleState{

terraform/test-fixtures/apply-module-destroy-order/child/main.tf

@@ -1,5 +1,7 @@
-resource "aws_instance" "a" {}
+resource "aws_instance" "a" {
+  id = "a"
+}
 
 output "a_output" {
-    value = "${aws_instance.a.id}"
+  value = "${aws_instance.a.id}"
 }

terraform/test-fixtures/apply-module-destroy-order/main.tf

@@ -3,5 +3,6 @@ module "child" {
 }
 
 resource "aws_instance" "b" {
-    blah = "${module.child.a_output}"
+  id   = "b"
+  blah = "${module.child.a_output}"
 }