refactor: improve test infrastructure and fix OpenGL GLAD initialization

- Rename D3D12Enum.h to D3D12Enums.h for naming consistency
- Fix OpenGL unit test GLAD initialization by using gladLoadGL()
  instead of gladLoadGLLoader(wglGetProcAddress) for fallback support
- Migrate remaining tests to use gtest_discover_tests for granular
  test discovery (math, core, containers, memory, threading, debug,
  components, scene, resources, input, opengl)
- Remove obsolete TEST_RESOURCES_DIR and copy_directory commands
  from OpenGL unit test CMakeLists (minimal/Res doesn't exist)
- Update TEST_SPEC.md with performance metrics and per-module
  build/test commands for faster development workflow
- Update CMake path references to use lowercase paths

tests/RHI/unit/CMakeLists.txt

@@ -34,8 +34,8 @@ target_include_directories(rhi_unit_tests PRIVATE
     ${CMAKE_CURRENT_SOURCE_DIR}/fixtures
     ${PROJECT_ROOT_DIR}/engine/include
     ${PROJECT_ROOT_DIR}/engine/src
-    ${CMAKE_SOURCE_DIR}/tests/OpenGL/package/include/
+    ${CMAKE_SOURCE_DIR}/tests/opengl/package/include/
 )
 
-enable_testing()
-add_test(NAME RHIUnitTests COMMAND rhi_unit_tests)
+include(GoogleTest)
+gtest_discover_tests(rhi_unit_tests)