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

engine/src/RHI/D3D12/D3D12CommandAllocator.cpp

@@ -1,5 +1,5 @@
 #include "XCEngine/RHI/D3D12/D3D12CommandAllocator.h"
-#include "XCEngine/RHI/D3D12/D3D12Enum.h"
+#include "XCEngine/RHI/D3D12/D3D12Enums.h"
 
 namespace XCEngine {
 namespace RHI {