bug(Autocomplete): unit test using MatAutocompleteHarness fails when karma browser is in the foreground

#### Reproduction

* Clone repo: https://github.com/DmitryEfimenko/mat-autocomplete-harness-issue
* install deps

Steps to reproduce:
1. In the console: `npm run test`
1. Make sure NOT to bring the opened Chrome window showing Jasmine tests into the visibility
If it opens in front, quickly click on some other window to bring the Chrome window to the background
1. Observe: test succeeds
1. make a small change in test (change spec name) to trigger test re-run
1. Observe: test still succeeds
1. Bring the Chrome window showing Jasmine tests into the visibility
1. Observe: test fail

Recording:
https://user-images.githubusercontent.com/2098175/113951456-76f5b680-97c8-11eb-9893-5cca66ac1a8c.mp4

#### Expected Behavior

Unit test should pass.
Specifically, `autocompleteHarness.getOptions()` should return 3 options instead of 0

#### Actual Behavior

Unit test only passes while the Jasmine Chrome window is in the background

#### Environment

- Angular: 11.2.8
- CDK/Material: 11.2.7
- Browser(s): Chrome
- Operating System (e.g. Windows, macOS, Ubuntu): Tested on:
- Windows
- Mac

#### Notes

I have created a StackBlitz repository with basically the same code, but can't reproduce the issue there:
https://stackblitz.com/edit/harness-issue-2m77t5?file=src/app/autocomplete-harness-example.spec.ts

Beginne mit dem verlinkten Reproduktions-Repository und führe `npm run test` aus. Reproduziere anschließend den Unterschied zwischen einem Chrome-Fenster im Hintergrund und im Vordergrund. Untersuche das Ergebnis von `autocompleteHarness.getOptions()`, das in beiden Fällen 3 Optionen enthalten sollte. Als abgeschlossen gilt die Aufgabe, wenn der Unit-Test konsistent besteht, unabhängig davon, ob das Jasmine-Chrome-Fenster sichtbar oder im Hintergrund ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.