Firestore system tests are flaky

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### Library Name

@google-cloud/firestore

### A screenshot that you have tested with "Try this API".

### Link to the code that reproduces this issue. A link to a **public** Github Repository or gist with a minimal reproduction.

https://fusion2.corp.google.com/ci/kokoro/prod:cloud-devrel%2Fclient-libraries%2Fnodejs%2Fpresubmit%2Fgoogleapis%2Fgoogle-cloud-node%2Fnodejs-firestore%2Fnode18%2Fsystem-test/activity/13806bf9-f026-4909-ac15-897ba1913d60/log

### A step-by-step description of how to reproduce the issue, based on the linked reproduction.

1. Create a PR
2. Run presubmits
3. Observe occasional failure of firestore system test

### A clear and concise description of what the bug is, and what you expected to happen.

The failing tests seem to have a variety of causes:
```
Error: 5 NOT_FOUND: No document to update: projects/java-review/databases/test-db/documents/foo/bar
```

```
"error": {
"code": 503,
"message": "Authentication backend unavailable.",
"errors": [
{
"message": "Authentication backend unavailable.",
"domain": "global",
"reason": "backendError"
}
],
"status": "UNAVAILABLE"
}
```
```
nc: connection failed, SOCKSv5 error: Host unreachable
Connection closed by UNKNOWN port 65535
rsync: connection unexpectedly closed (0 bytes received so far) [Receiver]
rsync error: unexplained error (code 255) at io.c(232) [Receiver=3.2.7]

java.io.IOException: Build failed with an internal error, check build logs for details.
```

### A clear and concise description WHY you expect this behavior, i.e., was it a recent change, there is documentation that points to this behavior, etc. **

These tests are failing for unrelated PRs, for example:
https://fusion2.corp.google.com/ci/kokoro/prod:cloud-devrel%2Fclient-libraries%2Fnodejs%2Fpresubmit%2Fgoogleapis%2Fgoogle-cloud-node%2Fnodejs-firestore%2Fnode18%2Fsystem-test/activity/30560954-62fb-46a9-af9e-1750cf9fb17b/details

https://fusion2.corp.google.com/ci/kokoro/prod:cloud-devrel%2Fclient-libraries%2Fnodejs%2Fpresubmit%2Fgoogleapis%2Fgoogle-cloud-node%2Fnodejs-firestore%2Fnode18%2Fsystem-test/activity/3122ab03-0d52-4bfd-9190-294feea6667d/details