pubsub: `run_exactly_once_roundtrip` test is flaky

The test just did not finish running:

```
test pubsub::run_exactly_once_roundtrip has been running for over 60 seconds
```

https://pantheon.corp.google.com/cloud-build/builds;region=us-central1/c16a5205-953b-4084-bfe7-748bcc3c580a;step=3?e=13803378&mods=allow_workbench_image_override&project=rust-sdk-testing#L22072

That is the last line in the log, 10 minutes into the build, and the build stops after 1 hour.

Start with the pubsub::run_exactly_once_roundtrip test named in the issue and reproduce it using the repository's test command. Inspect where this test waits or stops making progress, then verify the test completes reliably within the build timeout across repeated runs.

Written by the indexing model from the issue text.