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/D3D12/unit/test_command_list.cpp

@@ -1,11 +1,14 @@
 #include "fixtures/D3D12TestFixture.h"
+#include "XCEngine/RHI/D3D12/D3D12Enums.h"
+
+using namespace XCEngine::RHI;
 
 TEST_F(D3D12TestFixture, CommandList_Close_Basic) {
-    HRESULT hr = GetCommandList()->Close();
-    ASSERT_HRESULT_SUCCEEDED(hr);
+    GetCommandList()->Reset();
+    GetCommandList()->Close();
 }
 
 TEST_F(D3D12TestFixture, CommandList_Get_Desc) {
-    D3D12_COMMAND_LIST_TYPE type = GetCommandList()->GetType();
+    auto type = GetCommandList()->GetCommandList()->GetType();
     EXPECT_EQ(type, D3D12_COMMAND_LIST_TYPE_DIRECT);
-}
+}