feat(upgrade): provide unit test helpers for wiring up injectors (#16848)

Adds two new helper functions that can be used when unit testing Angular services
that depend upon upgraded AngularJS services, or vice versa.
The functions return a module (AngularJS or NgModule) that is configured to wire up
the Angular and AngularJS injectors without the need to actually bootstrap a full
hybrid application.

This makes it simpler and faster to unit test services.

PR Close #16848

packages/upgrade/static/testing/index.ts

@@ -0,0 +1,9 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+export * from './public_api';