Add Doctest unit testing to C++ template project

[theSlyest]

C++ Project Unit Testing

This PR integrates doctest (already part of the third-party dependencies) into the standard Axmol build system to enable seamless unit testing for C++ projects on desktop platforms.

Key changes include:

• CMake Architecture: Added a new option _AX_TESTS (default: OFF) in CMakeLists.txt. When enabled, the build system generates a separate test executable alongside the main game binary.
• Target Management: Configured set_target_properties to ensure the test executable is named ${APP_NAME}_test to prevent overwriting the game build.
• Entry Point Adaptation: Modified AppDelegate and desktop-specific main.cpp files to conditionally invoke the doctest runner when the test flag is active.
• Ensures that in "Test Mode," the engine initializes minimal requirements to run doctest suites instead of the main game loop.
• Configuration: Updated internal CMake configuration files to handle the inclusion of doctest headers and definitions globally when _AX_TESTS is set.

[gemini-code-assist]

Code Review

This pull request introduces a unit testing framework using doctest for the Axmol engine templates. It adds a new _AX_TESTS CMake option to enable a test target, modifies entry points across platforms (macOS, Linux, Windows) to run tests when enabled, and provides a sample test case. Feedback highlights a compilation error in the Windows entry point due to unnamed parameters, suggests using a generic bundle identifier for the test target on macOS, and recommends using a unique filename for the test batch script on Windows to avoid overwriting the main application's runner.

[theSlyest]

Hi @halx99, is there something I need to do?

[halx99]

Nothing, merged