expectObservable may silently cause a subscription / memory leak

Bug Report

Current Behavior
The TestScheduler:expectObservable internally subscribes to an observable, but the subscription may never be unsubscribed. This causes subscription/memory/state leaks in unit tests that are difficult to detect.

Reproduction

expectObservable(hot('-x-|')).toBe('-x-|'); // good
expectObservable(hot('-x-'), '1s !').toBe('-x-'); // good

expectObservable(hot('-x-')).toBe('-x-'); // leak
expectObservable(cold('-x-')).toBe('-x-'); // leak
expectObservable(NEVER).toBe(''); // leak

• https://stackblitz.com/edit/rxjs-expectobservable-memory-leak?file=test.ts

Expected behavior
All such subscriptions should be unsubscribed after completion of a test run.

Environment

• Runtime: [e.g. Node Any, Chrome Any]
• RxJS version: 6.5.4

Possible Solution
There are several options for how the issue can be fixed:

1. TestScheduler should silently unsubscribe from the observables after completion of the run.
2. TestScheduler should report a test failure if there are any observable still subscribed after completion of the run.
3. Add a new helper so a consumer can catch when the run is completed:

const source = helpers.hot('-x-').pipe(takeUntil(helpers.testEnd));
// const source = helpers.takeUntilTestEnd(helpers.hot('-x-'));
helpers.expectObservable(source).toBe('-x-');

Additional context/Screenshots
The possibility of leaks is mentioned at the page https://rxjs.dev/guide/testing/marble-testing#subscription-marbles.