Enable experimental feature for the test fixture

tests/modules/organization-policy/fixture/versions.tf

@@ -0,0 +1,20 @@
+# Copyright 2022 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+terraform {
+  required_version = ">= 1.1.0"
+
+  # TODO: Remove once Terraform 1.3 is released https://github.com/hashicorp/terraform/releases/tag/v1.3.0-alpha20220622
+  experiments = [module_variable_optional_attrs]
+}