Bump google-auth from 2.17.3 to 2.18.1 in /tests

Bumps [google-auth](https://github.com/googleapis/google-auth-library-python) from 2.17.3 to 2.18.1.
- [Release notes](https://github.com/googleapis/google-auth-library-python/releases)
- [Changelog](https://github.com/googleapis/google-auth-library-python/blob/main/CHANGELOG.md)
- [Commits](googleapis/google-auth-library-python@v2.17.3...v2.18.1)

---
updated-dependencies:
- dependency-name: google-auth
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

tests/requirements.txt

@@ -200,9 +200,9 @@ flaky==3.7.0 \
 forcediphttpsadapter==1.0.2 \
     --hash=sha256:f7582b34aaa6ab6b17f69ab1abbfe67097b952ed0682b758b5e01e18fe24433e
     # via -r requirements.txt
-google-auth==2.17.3 \
-    --hash=sha256:ce311e2bc58b130fddf316df57c9b3943c2a7b4f6ec31de9663a9333e4064efc \
-    --hash=sha256:f586b274d3eb7bd932ea424b1c702a30e0393a2e2bc4ca3eae8263ffd8be229f
+google-auth==2.18.1 \
+    --hash=sha256:55a395cdfd3f3dd3f649131d41f97c17b4ed8a2aac1be3502090c716314e8a37 \
+    --hash=sha256:d7a3249027e7f464fbbfd7ee8319a08ad09d2eea51578575c4bd360ffa049ccb
     # via
     #   -r requirements.txt
     #   kubernetes