Flaky rust-sdk example

[laurentsenta]

Version information:

Current master

Description:

With the rust-sdk: json payload fix, we added a second test and the GitHub action appears to have become (more) flaky.

See: https://github.com/testground/sdk-rust/actions?query=branch%3Afix%2Fissue-33-json-payloads
It seems the test started failing after adding publish/subscribe to the example.
You have to drill down the runs, but we get approx. 2 / 6 attempts passing with the example case.