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/OpenGL/integration/triangle/CMakeLists.txt

@@ -5,7 +5,7 @@ set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
 project(OpenGL_Triangle)
 
 set(ENGINE_ROOT_DIR ${CMAKE_SOURCE_DIR}/engine)
-set(PACKAGE_DIR ${CMAKE_SOURCE_DIR}/tests/OpenGL/package)
+set(PACKAGE_DIR ${CMAKE_SOURCE_DIR}/tests/opengl/package)
 
 add_executable(OpenGL_Triangle
     WIN32
@@ -37,10 +37,10 @@ add_custom_command(TARGET OpenGL_Triangle POST_BUILD
 
 add_custom_command(TARGET OpenGL_Triangle POST_BUILD
     COMMAND ${CMAKE_COMMAND} -E copy_if_different
-        ${CMAKE_SOURCE_DIR}/tests/RHI/OpenGL/integration/run_integration_test.py
+        ${CMAKE_SOURCE_DIR}/tests/rhi/opengl/integration/run_integration_test.py
         $<TARGET_FILE_DIR:OpenGL_Triangle>/
     COMMAND ${CMAKE_COMMAND} -E copy_if_different
-        ${CMAKE_SOURCE_DIR}/tests/RHI/OpenGL/integration/compare_ppm.py
+        ${CMAKE_SOURCE_DIR}/tests/rhi/opengl/integration/compare_ppm.py
         $<TARGET_FILE_DIR:OpenGL_Triangle>/
     COMMAND ${CMAKE_COMMAND} -E copy_if_different
         ${CMAKE_CURRENT_SOURCE_DIR}/GT.ppm