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.

Commencez par la reproduction StackBlitz liée, en particulier src/app/example-test.spec.ts, et suivez MatSnackBarHarness.getMessage() lorsque la snack bar a une durée. Comparez-la à une snack bar sans durée. Le travail est terminé lorsque le harness récupère le message au lieu d’attendre la fermeture et d’échouer avec "Failed to find element".

Rédigé par le modèle d'indexation à partir du texte de l'issue.