bug(MatSnackBarHarness): Testing a snack bar with a duration

#### Reproduction

https://stackblitz.com/edit/components-issue-v9512z?file=src%2Fapp%2Fexample-test.spec.ts

Steps to reproduce:
1. Try to e.g. `await harness.getMessage()` a snack bar that was opened with a `duration`. You can see this simply by following the link above, which will run that automatically.

#### Expected Behavior

The harness returns the message in the snack bar.

#### Actual Behavior

The test hangs for the duration that the snack bar is open, then fails with "Failed to find element".

#### Environment

- Angular: 9.1.5
- CDK/Material: 9.2.2
- Browser(s): Chrome
- Operating System (e.g. Windows, macOS, Ubuntu): Windows

#### Commentary

Using the same test code, I can test a snack bar that was opened without a duration. But if my production code _does_ use a duration it does not appear possible to test it with the harness. Am I missing something? I assume assigning a duration is a common use-case.

Inizia dalla riproduzione StackBlitz collegata, in particolare src/app/example-test.spec.ts, e segui MatSnackBarHarness.getMessage() quando la snack bar ha una durata. Confrontala con una snack bar senza durata. Il lavoro è completo quando l’harness recupera il messaggio invece di attendere la chiusura e fallire con "Failed to find element".

Scritto dal modello di indicizzazione a partire dal testo della issue.