cleanup: migrate to TestBed.inject from TestBed.get

[stephanwlee]

Angular 9.0 deprecated TestBed.get in favor of TestBed.inject. Migrate to the next API.

Context:

• https://angular.io/api/core/testing/TestBed
• https://v8.angular.io/api/core/testing/TestBed (v8 version had only .get).

[stephanwlee]

Note that this is blocked by #3232.