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/fixtures/OpenGLTestFixture.h

@@ -0,0 +1,49 @@
+#pragma once
+
+#include <glad/glad.h>
+#include <GLFW/glfw3.h>
+#include <gtest/gtest.h>
+
+#include "XCEngine/RHI/OpenGL/OpenGLDevice.h"
+
+namespace XCEngine {
+namespace RHI {
+
+class OpenGLTestFixture : public ::testing::Test {
+protected:
+    void SetUp() override;
+    void TearDown() override;
+
+    GLFWwindow* GetWindow() { return m_window; }
+    void MakeContextCurrent();
+    void DoneContextCurrent();
+
+    void ClearGLErrors();
+    bool CheckGLError(const char* file, int line);
+    const char* GetGLErrorString(GLenum error);
+    void ResetGLState();
+
+private:
+    GLFWwindow* m_window = nullptr;
+    OpenGLDevice* m_device = nullptr;
+};
+
+} // namespace RHI
+} // namespace XCEngine
+
+#define GL_CLEAR_ERRORS() \
+    do { while (glGetError() != GL_NO_ERROR); } while(0)
+
+#define GL_CHECK(call) \
+    do { \
+        call; \
+        ASSERT_TRUE(XCEngine::RHI::OpenGLTestFixture::CheckGLError(__FILE__, __LINE__)) \
+            << "GL error after " << #call; \
+    } while(0)
+
+#define GL_EXPECT_SUCCESS(call) \
+    do { \
+        call; \
+        EXPECT_EQ(glGetError(), GL_NO_ERROR) \
+            << "GL error after " << #call; \
+    } while(0)