Add Phase 1 D3D12 tests for Device and Fence

- Implement real D3D12 tests for Device (feature level, descriptor handle,
  shader model, resource binding tier, tiled resources)
- Implement real D3D12 tests for Fence (create, signal, wait, event)
- Move tests from tests/D3D12_engine/test/ to tests/RHI/D3D12/
- All 22 tests pass

tests/RHI/D3D12/test_descriptor_heap.cpp

@@ -0,0 +1,5 @@
+#include "fixtures/D3D12TestFixture.h"
+
+TEST_F(D3D12TestFixture, DescriptorHeap_Placeholder) {
+    ASSERT_TRUE(true);
+}