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_vertex_array.cpp

@@ -0,0 +1,70 @@
+#include "fixtures/OpenGLTestFixture.h"
+#include "XCEngine/RHI/OpenGL/OpenGLVertexArray.h"
+#include "XCEngine/RHI/OpenGL/OpenGLBuffer.h"
+
+using namespace XCEngine::RHI;
+
+TEST_F(OpenGLTestFixture, VertexArray_Initialize_CreatesVAO) {
+    OpenGLVertexArray vao;
+    
+    bool result = vao.Initialize();
+    
+    ASSERT_TRUE(result);
+    EXPECT_NE(vao.GetID(), 0u);
+    
+    vao.Shutdown();
+}
+
+TEST_F(OpenGLTestFixture, VertexArray_AddVertexBuffer) {
+    OpenGLVertexArray vao;
+    vao.Initialize();
+    
+    OpenGLBuffer buffer;
+    buffer.InitializeVertexBuffer(nullptr, 64);
+    
+    VertexAttribute attr;
+    attr.index = 0;
+    attr.count = 3;
+    attr.type = GL_FLOAT;
+    attr.normalized = false;
+    attr.stride = sizeof(float) * 3;
+    attr.offset = 0;
+    
+    vao.AddVertexBuffer(buffer.GetID(), attr);
+    
+    EXPECT_EQ(vao.GetID(), 1u);
+    
+    buffer.Shutdown();
+    vao.Shutdown();
+}
+
+TEST_F(OpenGLTestFixture, VertexArray_SetIndexBuffer) {
+    OpenGLVertexArray vao;
+    vao.Initialize();
+    
+    OpenGLBuffer buffer;
+    buffer.InitializeIndexBuffer(nullptr, 12);
+    
+    vao.SetIndexBuffer(buffer.GetID(), GL_UNSIGNED_INT);
+    
+    EXPECT_NE(vao.GetIndexBuffer(), 0u);
+    
+    buffer.Shutdown();
+    vao.Shutdown();
+}
+
+TEST_F(OpenGLTestFixture, VertexArray_Bind_Unbind) {
+    OpenGLVertexArray vao;
+    vao.Initialize();
+    
+    vao.Bind();
+    GLint boundVAO = 0;
+    glGetIntegerv(GL_VERTEX_ARRAY_BINDING, &boundVAO);
+    EXPECT_EQ(boundVAO, static_cast<GLint>(vao.GetID()));
+    
+    vao.Unbind();
+    glGetIntegerv(GL_VERTEX_ARRAY_BINDING, &boundVAO);
+    EXPECT_EQ(boundVAO, 0);
+    
+    vao.Shutdown();
+}