How to test concatenate streams in rxjs

Question

The given class has a method which returns a cached stream but that stream can be triggered by another private hot stream which makes the cached stream emits a new value.

The class

export class SomeClass {
  private cache: Observable<number>;
  private trigger$ = new Subject();
  private multiply = 1;

  constructor(private num: number) {}

  getNumber(): Observable<number> {
    return (
      this.cache ||
      (this.cache = concat(of(void 0), this.trigger$).pipe(
        switchMap(() => of(this.num * this.multiply++)),
        shareReplay(1)
      ))
    );
  }

  trigger(): void {
    this.trigger$.next();
  }
}

Example: https://stackblitz.com/edit/rxjs-gpyc46?file=index.ts

What is the way to test it? This try is failed

it("trigger updates", () => {
  testScheduler.run(({ expectObservable }) => {
    const num$ = someClass.getNumber();
    expectObservable(num$).toBe("a", { a: 3 });
    someClass.trigger();
    expectObservable(num$).toBe("a", { a: 6 });
    someClass.trigger();
    expectObservable(num$).toBe("a", { a: 9 });
  });
});

Example: https://stackblitz.com/edit/rxjs-test-tricky-flow?file=src%2Fsome-class.spec.ts

UPD: so the problem here that seems it is not possible to mock the trigger$ property.

It would look like this

it("trigger updates", () => {
  testScheduler.run(({ hot, expectObservable }) => {
    spyOnProperty(someClass, 'trigger$', 'get').and.returnValue(hot('^--b--c'));
    const num$ = someClass.getNumber();
    expectObservable(num$).toBe("a--b--c", { a: 3, b: 6, c: 9 });
  });
});

But the trigger$ property must be changed this way

  get trigger$() {
    return new Subject();
  }

Example: https://stackblitz.com/edit/rxjs-test-tricky-flow-x2arxf?file=src%2Fsome-class.ts