feat: add RHI abstraction layer unit tests

- Add RHITestFixture with RHI_BACKEND env var support for backend selection
- Add unit tests for: Device, Buffer, Texture, SwapChain, CommandList, CommandQueue, Shader, Fence, Sampler
- Tests can run against D3D12 or OpenGL backends via RHI_BACKEND env var
- Add integration folder placeholder for future integration tests

tests/RHI/CMakeLists.txt

@@ -41,3 +41,5 @@ target_include_directories(rhi_engine_tests PRIVATE
 
 enable_testing()
 add_test(NAME RHIEngineTests COMMAND rhi_engine_tests)
+
+add_subdirectory(unit)