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

aio/tools/transforms/angular-api-package/index.js

@@ -115,6 +115,7 @@ module.exports =
             'service-worker/index.ts',
             'upgrade/index.ts',
             'upgrade/static/index.ts',
+            'upgrade/static/testing/index.ts',
           ];
 
           readFilesProcessor.fileReaders.push(packageContentFileReader);