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/unit/CMakeLists.txt

@@ -0,0 +1,41 @@
+cmake_minimum_required(VERSION 3.15)
+
+get_filename_component(PROJECT_ROOT_DIR ${CMAKE_CURRENT_SOURCE_DIR}/../../../.. ABSOLUTE)
+
+find_package(GTest REQUIRED)
+
+set(TEST_SOURCES
+    fixtures/RHITestFixture.cpp
+    test_device.cpp
+    test_buffer.cpp
+    test_texture.cpp
+    test_swap_chain.cpp
+    test_command_list.cpp
+    test_command_queue.cpp
+    test_shader.cpp
+    test_fence.cpp
+    test_sampler.cpp
+)
+
+add_executable(rhi_unit_tests ${TEST_SOURCES})
+
+target_link_libraries(rhi_unit_tests PRIVATE
+    d3d12
+    dxgi
+    d3dcompiler
+    opengl32
+    glfw3
+    XCEngine
+    GTest::gtest
+    GTest::gtest_main
+)
+
+target_include_directories(rhi_unit_tests PRIVATE
+    ${CMAKE_CURRENT_SOURCE_DIR}/fixtures
+    ${PROJECT_ROOT_DIR}/engine/include
+    ${PROJECT_ROOT_DIR}/engine/src
+    ${CMAKE_SOURCE_DIR}/tests/OpenGL/package/include/
+)
+
+enable_testing()
+add_test(NAME RHIUnitTests COMMAND rhi_unit_tests)