chore(deps): update dependency google-cloud-testutils to v1.3.1 (#359)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [google-cloud-testutils](https://togithub.com/googleapis/python-test-utils) | `==1.3.0` -> `==1.3.1` | [![age](https://badges.renovateapi.com/packages/pypi/google-cloud-testutils/1.3.1/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/google-cloud-testutils/1.3.1/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/google-cloud-testutils/1.3.1/compatibility-slim/1.3.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/google-cloud-testutils/1.3.1/confidence-slim/1.3.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>googleapis/python-test-utils</summary>

### [`v1.3.1`](https://togithub.com/googleapis/python-test-utils/blob/master/CHANGELOG.md#&#8203;131-httpswwwgithubcomgoogleapispython-test-utilscomparev130v131-2021-12-07)

[Compare Source](https://togithub.com/googleapis/python-test-utils/compare/v1.3.0...v1.3.1)

</details>

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packages/google-cloud-bigquery-storage/samples/snippets/requirements-test.txt

@@ -1,2 +1,2 @@
-google-cloud-testutils==1.3.0
+google-cloud-testutils==1.3.1
 pytest==6.2.5