core: Mock schema for TestContext2Apply_multiProviderDestroy

This test seems to have been buggy before our current work, with the test
fixture containing a reference to a resource that doesn't exist.

This both fixes the fixture and adds a mock schema for it, though this
just revealed another error which isn't fixed here, where the a_ids value
seems to come through as unknown after apply. That will be fixed in a
subsequent commit.

terraform/context_apply_test.go

@@ -3900,6 +3900,16 @@ func TestContext2Apply_multiVarMissingState(t *testing.T) {
 	p := testProvider("test")
 	p.ApplyFn = testApplyFn
 	p.DiffFn = testDiffFn
+	p.GetSchemaReturn = &ProviderSchema{
+		ResourceTypes: map[string]*configschema.Block{
+			"test_thing": {
+				Attributes: map[string]*configschema.Attribute{
+					"a_ids": {Type: cty.String, Optional: true},
+					"id":    {Type: cty.String, Computed: true},
+				},
+			},
+		},
+	}
 
 	// First, apply with a count of 3
 	ctx := testContext2(t, &ContextOpts{

terraform/test-fixtures/apply-multi-var-missing-state/child/child.tf

@@ -11,5 +11,5 @@ resource "test_thing" "a" {
 # over all of the resource in the state. This should succeed even though the
 # module state will be nil when evaluating the variable.
 resource "test_thing" "b" {
-  a_ids = "${join(" ", null_resource.a.*.id)}"
+  a_ids = "${join(" ", test_thing.a.*.id)}"
 }