Expose reusable test harness libraries

[zayenz]

Summary

• Exposes the Gecode test runner and the helpers for every configured variable family as reusable static libraries.
• Installs Gecode::gecodetestint, Gecode::gecodetestset, and Gecode::gecodetestfloat with their public headers. Set and float follow the corresponding build options.
• Provides the same target names for installed packages, add_subdirectory, and FetchContent, with matching Autoconf archives.
• Documents custom integer, Boolean, set, and float propagator tests, including independent oracles, consistency controls, reification, and failure reproduction.
• Records the public test libraries in the Gecode 6.5.0 changelog.

Validation

• CMake runner, integer, set, and float smoke tests
• Full and integer-only CMake configurations
• Full gecode-test link after extracting the helper objects
• Installed CMake consumers for full and integer-only packages
• Parallel CTest fixture execution
• Fault-injection test target
• Absolute include directory and customized library names
• BUILD_TESTING=TRUE package discovery
• Build-tree target aliases
• Autoconf smoke tests and installed native consumer
• Unsupported CMake component and internal legacy header diagnostics
• git diff --check