`sample_event.json` files don't include `constant_keyword` fields

[chrisberkhout]

Constant keyword fields aren't included in the sample_event.json files generated by system tests. Looking in Elasticsearch at the end of the test run does show such fields.

For example, in elastic/integrations#14017 the field event.module was added in the events data stream, but it's not present in the regenerated sample_event.json file.

[jsoriano]

Thanks for reporting. I think we get the sample event from the _source, and I think constant_keywords don't appear there, we should also check fields, as we do when verifying fields in some tests.