feat(upgrade): enable Angular 1 unit testing of upgrade module

- New method `UpgradeAdapter.registerForNg1Tests(modules)` declares the
  Angular 1 upgrade module and provides it to the `angular.mock.module()`
  helper.
  This prevents the need to bootstrap the entire hybrid for every test.

Closes #5462, #12675

tools/public_api_guard/upgrade/index.d.ts

@@ -4,6 +4,7 @@ export declare class UpgradeAdapter {
     bootstrap(element: Element, modules?: any[], config?: angular.IAngularBootstrapConfig): UpgradeAdapterRef;
     downgradeNg2Component(type: Type<any>): Function;
     downgradeNg2Provider(token: any): Function;
+    registerForNg1Tests(modules?: string[]): UpgradeAdapterRef;
     upgradeNg1Component(name: string): Type<any>;
     upgradeNg1Provider(name: string, options?: {
         asToken: any;