Fix: resolve intermittent system-test timeouts in common.ts

[thiyaguk09]

• Implement dynamic port allocation (port 0) for mock servers to avoid EADDRINUSE collisions in CI.
• Increase timeout to 90s for retry logic to accommodate environmental latency.
• Ensure mock servers are explicitly closed in both success and failure blocks.
• Fix async race condition in 'non-responsive hosts' test by moving done() into the callback.

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Fixes #7343

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