fix: TestAsyncTransport_SendEnvelope flakiness

[giortzisg]

Description

The queue overflow testcase was using an unbuffered channel for RequestReceived, to block the worker so that no other events get consumed and the overflow will be guaranteed. The channel however was unbuffered, so the requestReceived (worker) could race with the test and block the main routine till timeout

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Issues

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Semver Impact of This PR

🟢 Patch (bug fixes)

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