Add health monitoring tests for draining writers

- Introduced adversarial tests to validate the behavior of the health monitoring system under various conditions, including the management of draining writers.
- Implemented integration tests to ensure the health monitor correctly handles expired and empty draining writers.
- Added regression tests to verify the functionality of the draining writers' cleanup process, ensuring it adheres to the defined thresholds and budgets.
- Updated the module structure to include the new test files for better organization and maintainability.

src/transport/middle_proxy/mod.rs

@@ -21,6 +21,12 @@ mod secret;
 mod selftest;
 mod wire;
 mod pool_status;
+#[cfg(test)]
+mod health_regression_tests;
+#[cfg(test)]
+mod health_integration_tests;
+#[cfg(test)]
+mod health_adversarial_tests;
 
 use bytes::Bytes;