Add tests - First round

[koudis]

Summary by CodeRabbit

• Bug Fixes

  • Corrected the flag for macOS application bundle creation to use the proper MACOSX_BUNDLE option when adding executables on macOS platforms.

• Style

  • Fixed typographical errors and improved clarity in comments and documentation across multiple files.

• Documentation

  • Updated documentation for cache variables and package chain usage, enhancing explanations and correcting minor errors.
  • Refined README files for core components and test suites to clarify usage and structure.

• Chores

  • Added extensive new test suites covering executables, libraries, environment detection, installation, and packaging features.
  • Introduced helper macros and scripts for test validation, installation interception, cache variable management, and package verification.
  • Added a GitHub Actions workflow for automated cross-platform testing on Linux (multiple distros), macOS, and Windows.