Add missing copyright headers

Author: hashicc

addons/api/addon-test-support/helpers/mirage.js

@@ -1,3 +1,8 @@
+/**
+ * Copyright (c) HashiCorp, Inc.
+ * SPDX-License-Identifier: BUSL-1.1
+ */
+
 import { settled } from '@ember/test-helpers';
 import startMirage from 'api/mirage/config';