fix: use gtest_discover_tests for granular CTest test discovery

Replace add_test() with gtest_discover_tests() to enable CTest to
discover all 55 individual Google Test cases instead of treating them
as a single test. This improves test reporting granularity from 1/1
to 55/55 tests.

tests/RHI/D3D12/unit/CMakeLists.txt

@@ -41,4 +41,5 @@ target_include_directories(d3d12_engine_tests PRIVATE
 
 
 enable_testing()
-add_test(NAME D3D12EngineTests COMMAND d3d12_engine_tests)
+include(GoogleTest)
+gtest_discover_tests(d3d12_engine_tests)