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_shader.cpp

@@ -1,5 +1,7 @@
 #include "fixtures/D3D12TestFixture.h"
 
+using namespace XCEngine::RHI;
+
 TEST_F(D3D12TestFixture, Shader_Get_VertexShaderProfile) {
     const char* profile = "vs_6_0";
     EXPECT_STREQ(profile, "vs_6_0");