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

tests/RHI/D3D12/integration/CMakeLists.txt

@@ -102,7 +102,7 @@ add_test(NAME D3D12_Minimal_Integration
     COMMAND ${Python3_EXECUTABLE} $<TARGET_FILE_DIR:D3D12_Minimal>/run_integration_test.py
         $<TARGET_FILE:D3D12_Minimal>
         minimal.ppm
-        ${CMAKE_CURRENT_SOURCE_DIR}/minimal/GT_minimal.ppm
+        ${CMAKE_CURRENT_SOURCE_DIR}/minimal/GT.ppm
         5
     WORKING_DIRECTORY $<TARGET_FILE_DIR:D3D12_Minimal>
 )