Don't access private context when test data is needed in the Extractor

[bytinbit]

napalm_ping.py (and iperf too) needs to access a context's test_data from within the Extractor, which is not nice:

test_data: List[Dict[str, Any]] = self._nuts_ctx.nuts_parameters["test_data"].

Find a solution to pass the entry into single_transform instead, so it can be used there.