OpenGL: Restructure tests similar to D3D12 layout

- Move old test files to new unit/integration structure
- Add OpenGL Test Fixture
- Update CMakeLists.txt for new layout
- Add OpenGL_Test_Restructuring_Plan.md

tests/RHI/OpenGL/unit/test_swap_chain.cpp

@@ -0,0 +1,41 @@
+#include "fixtures/OpenGLTestFixture.h"
+#include "XCEngine/RHI/OpenGL/OpenGLSwapChain.h"
+
+using namespace XCEngine::RHI;
+
+TEST_F(OpenGLTestFixture, SwapChain_Initialize_Window) {
+    OpenGLSwapChain swapChain;
+    GLFWwindow* window = GetWindow();
+    
+    bool result = swapChain.Initialize(window, 800, 600);
+    
+    ASSERT_TRUE(result);
+    
+    swapChain.Shutdown();
+}
+
+TEST_F(OpenGLTestFixture, SwapChain_Present) {
+    OpenGLSwapChain swapChain;
+    GLFWwindow* window = GetWindow();
+    swapChain.Initialize(window, 800, 600);
+    
+    swapChain.Present();
+    
+    GLenum error = glGetError();
+    EXPECT_EQ(error, GL_NO_ERROR);
+    
+    swapChain.Shutdown();
+}
+
+TEST_F(OpenGLTestFixture, SwapChain_Resize_ChangesSize) {
+    OpenGLSwapChain swapChain;
+    GLFWwindow* window = GetWindow();
+    swapChain.Initialize(window, 800, 600);
+    
+    swapChain.Resize(1024, 768);
+    
+    EXPECT_EQ(swapChain.GetWidth(), 1024);
+    EXPECT_EQ(swapChain.GetHeight(), 768);
+    
+    swapChain.Shutdown();
+}