Update Go test files to use run/Runner terminology
Update test config strings and assertions across Go test packages:
- pkg/config/: config_test, load_test, validate_test use run: key
- pkg/dockerfile/: standard_generator_test uses run: 'run.py:Runner'
- pkg/util/: overwrite_yaml_test uses run: key
- pkg/web/: client_test creates run.py temp files
Note: Docker image label JSON continues to use 'predict' key (part of
Config struct's json tag), so model/image tests and mock configs that
test label parsing are intentionally unchanged.
