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/core/CMakeLists.txt

@@ -6,25 +6,26 @@ set(CORE_TEST_SOURCES
     test_core.cpp
 )
 
-add_executable(xcengine_core_tests ${CORE_TEST_SOURCES})
+add_executable(core_tests ${CORE_TEST_SOURCES})
 
 # Exclude all static runtime libraries to avoid conflicts
 if(MSVC)
-    set_target_properties(xcengine_core_tests PROPERTIES
+    set_target_properties(core_tests PROPERTIES
         LINK_FLAGS "/NODEFAULTLIB:libcpmt.lib /NODEFAULTLIB:libcmt.lib"
     )
 endif()
 
-target_link_libraries(xcengine_core_tests
+target_link_libraries(core_tests
     PRIVATE
     XCEngine
     GTest::gtest
     GTest::gtest_main
 )
 
-target_include_directories(xcengine_core_tests PRIVATE
+target_include_directories(core_tests PRIVATE
     ${CMAKE_SOURCE_DIR}/engine/include
     ${CMAKE_SOURCE_DIR}/tests/fixtures
 )
 
-add_test(NAME CoreTests COMMAND xcengine_core_tests)
+include(GoogleTest)
+gtest_discover_tests(core_tests)