Rename GT_minimal.ppm to GT.ppm for D3D12 minimal integration test

- Rename golden image file to match simpler convention
- Update CMakeLists.txt reference
- Update TEST_SPEC.md documentation
- Update run_tests.py reference

scripts/run_tests.py

@@ -275,12 +275,7 @@ class TestRunner:
 
         minimal_exe = integration_dir / "D3D12_Minimal.exe"
         gt_minimal = (
-            self.source_dir
-            / "tests"
-            / "RHI"
-            / "D3D12"
-            / "integration"
-            / "GT_minimal.ppm"
+            self.source_dir / "tests" / "RHI" / "D3D12" / "integration" / "GT.ppm"
         )
 
         if minimal_exe.exists() and gt_minimal.exists():