OpenGL: Refactor integration test and enable CTest framework

- Simplify OpenGL integration test structure
- Enable CTest registration for OpenGL tests
- Refactor test fixtures and device enumeration
- Minor code cleanup and improvements

tests/RHI/OpenGL/unit/test_device.cpp

@@ -23,7 +23,7 @@ TEST_F(OpenGLTestFixture, Device_CreateRenderWindow_DebugMode) {
 
 TEST_F(OpenGLTestFixture, Device_InitializeWithExistingWindow) {
     OpenGLDevice device;
-    GLFWwindow* existingWindow = GetWindow();
+    HWND existingWindow = GetWindow();
 
     bool result = device.InitializeWithExistingWindow(existingWindow);