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

tools/public_api_guard/upgrade/static/testing.d.ts

@@ -0,0 +1,3 @@
+export declare function createAngularJSTestingModule(angularModules: any[]): string;
+
+export declare function createAngularTestingModule(angularJSModules: string[], strictDi?: boolean): Type<any>;