Fix Makefile of testint plugin

[tchssk]

This pull request refactors and improves the testing plugin’s example management and build process, updates generated code to reflect newer tool versions, and introduces better linting and exclusion practices. The changes make it easier to generate, build, and test example services, and ensure that generated files and linting processes are up-to-date and accurate.

Build and Example Management Improvements:

• Major refactor of testing/Makefile to add explicit gen, example, test, build-examples, and clean targets for generating, building, and testing example services (calculator, httpgrpc, jsonrpc). This replaces the old, less flexible approach and provides clearer, more maintainable commands.
• Addition of a .golint_exclude file to exclude the examples directory from linting, preventing unnecessary lint errors on example code.

Linting and Formatting Enhancements:

• The lint target in plugins.mk is updated to exclude .pb.go files (protobuf-generated code) from formatting checks, avoiding false positives and unnecessary formatting errors.

Generated Code Updates:

• All generated files in testing/examples/calculator/gen and related subdirectories now show updated headers, reflecting the latest version of goa and the new command-line arguments used for code generation. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
• The goa.json file is added to record the goa version used for code generation, supporting better reproducibility.

Minor Documentation and Code Quality:

• Minor improvements to comments and docstrings for clarity and formatting in test harness code.
• Minor improvements to error name documentation for generated error types.

These changes collectively improve the maintainability, reliability, and developer experience when working with the testing plugin and its example services.