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/sphere/Res/Shader/sphere.vert

@@ -0,0 +1,17 @@
+#version 460
+
+layout(location = 0) in vec4 aPosition;
+layout(location = 1) in vec2 aTexcoord;
+
+out vec2 vTexcoord;
+
+uniform mat4 gModelMatrix;
+uniform mat4 gViewMatrix;
+uniform mat4 gProjectionMatrix;
+
+void main() {
+    vec4 positionWS = gModelMatrix * aPosition;
+    vec4 positionVS = gViewMatrix * positionWS;
+    gl_Position = gProjectionMatrix * positionVS;
+    vTexcoord = aTexcoord;
+}