feat: Add comprehensive tests and ensure Go 1.22.2 compatibility

[maurice2k]

This commit introduces a significant number of improvements to the ab-proxy tool:

1. Go Version Compatibility:

   • Ensured the project builds and runs correctly with Go 1.22.2.
   • Updated go.mod to specify go 1.22.2.
   • Dependencies were verified to be compatible, and go.sum was updated.

2. Refactoring for Testability:

   • The main application logic in ab-proxy.go was refactored.
   • Core benchmarking functionality was moved into a runBenchmark function.
   • Global state for options, statistics, and error collection was encapsulated into structs (Options, Stats, ErrorCollector), making the code more modular and testable.

3. Unit Tests:

   • Added extensive unit tests in ab_proxy_test.go.
   • These tests cover argument parsing, default value handling, header preparation, proxy configuration logic, URL validation, and the ErrorCollector functionality.

4. Integration Tests:

   • Introduced mock HTTP target and HTTP proxy servers (mockservers_test.go) to simulate real-world scenarios.
   • Added a comprehensive suite of integration tests in ab_proxy_integration_test.go.
   • These tests execute ab-proxy as a subprocess and verify its behavior against the mock servers, covering:
     • Basic GET requests and statistics reporting.
     • Custom User-Agent and header propagation.
     • Client-side timeout functionality.
     • HTTP proxying.
     • Handling of target server errors (e.g., HTTP 500).
     • Error reporting for connection failures (e.g., connection refused) using --show-errors.
     • Functionality of multiple bursts and delays.

5. Dockerization for Testing:

   • Added a Dockerfile that creates a containerized environment with Go 1.22.2.
   • The Dockerfile is configured to build the application and run all unit and integration tests.
   • All tests were confirmed to pass within this Docker environment.

These changes enhance the reliability and maintainability of ab-proxy by providing a strong testing foundation and ensuring compatibility with an updated Go version.