Refactor test_zone tests to make them run independently

[shhovhan]

Summary

This PR refactors the DNS functional tests to make them fully independent and resilient to resource quota limits.

Key Improvements

1. Test Independence
2. Removed shared state across tests (e.g. class-level networks)
3. Each test now creates and manages its own resources
4. Tests no longer depend on execution order
5. Robust Cleanup in tearDown()
   • Ensures cleanup always runs using try/finally
   • Deletes resources in reverse order for safety
   • Adds new cleanup method for deleting routers that are not removed for any reason to avoid Quota exceeded errors

Result

1. Tests can now run independently and in any order
2. Significantly reduced risk of quota exhaustion
3. Improved stability in CI environments
4. More accurate validation of real system behavior

[otc-zuul]

Build succeeded.
https://zuul.otc-service.com/t/eco/buildset/72d3520961f4458d998e09db0d780cfa

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