Stop updating event.ingested in -expected.json files

For testing log data streams, we usually have sample logs under `_dev/test` directory and run `elastic-package test pipeline --generate` command to generate the `-expected.json` files. These `-expected.json` files all have the `event.ingested` field which gets updated every time with a new timestamp. But this way, even with changes in the ingest pipeline or fields.yml that are not related to some logs, the `-expected.json` file will still be updated with the only difference is the `event.ingested` timestamp.

Maybe we can disable updating this field when running `elastic-package test pipeline --generate` so it's easier to review the `-expected.json` files in PRs that are actually impacted by changes you made in the ingest pipeline.

Start by tracing the `elastic-package test pipeline --generate` command and its handling of sample logs under `_dev/test`. Check where `-expected.json` files are generated and verify that unrelated runs no longer update `event.ingested`, while changes to other expected fields remain visible.

Written by the indexing model from the issue text.