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

[thiyaguk09]

Description

This PR resolves intermittent flakiness and timeouts in the Service system tests within common.ts.

Key Changes:

• Dynamic Ports: Refactored mock servers to use port 0 (ephemeral ports) instead of hardcoded 8118 to prevent EADDRINUSE collisions in CI runners.
• Retry Logic: Increased the test timeout to 90s to accommodate exponential backoff delays and CI environment latency.
• Lifecycle Management: Wrapped assertions in try/catch to ensure mockServer.close() is always called, preventing socket leaks.
• Async Fix: Corrected a race condition in the non-responsive host test where done() was being called before the retry cycle completed.

Impact

Reduces "Exit Status 1" build failures in Kokoro CI, improving the stability of the continuous integration pipeline for the Node.js Storage library.

Testing

• System Tests: Verified locally via npm run system-test.
• Telemetry: Inspected retry cycles using NODE_DEBUG=http to confirm the library correctly registers all 4 attempts before concluding.
• Cleanup: Confirmed no orphaned processes or open sockets remain after test completion.

Additional Information

The original 60s timeout was frequently exceeded during the 4-attempt retry cycle when the CI runner was under heavy load. The move to dynamic ports is the primary structural fix to prevent cross-test interference.

Checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease
• Appropriate docs were updated
• Appropriate comments were added, particularly in complex areas or places that require background
• No new warnings or issues will be generated from this change

Fixes #2647