修复D3D12SwapChain初始化bug并添加单元测试

- 修复Initialize(IDXGIFactory4*, ...)重载缺少m_backBuffers初始化的问题
- 新增test_swap_chain.cpp单元测试文件，包含6个SwapChain测试用例
- 更新unit CMakeLists.txt添加test_swap_chain.cpp和Res路径

engine/src/RHI/D3D12/D3D12SwapChain.cpp

@@ -44,6 +44,13 @@ bool D3D12SwapChain::Initialize(IDXGIFactory4* factory, ID3D12CommandQueue* comm
     m_height = height;
     m_bufferCount = bufferCount;
 
+    m_backBuffers.resize(m_bufferCount);
+    for (uint32_t i = 0; i < m_bufferCount; ++i) {
+        ID3D12Resource* resource = nullptr;
+        m_swapChain->GetBuffer(i, IID_PPV_ARGS(&resource));
+        m_backBuffers[i].InitializeFromExisting(resource);
+    }
+
     return true;
 }