fix(elasticsearch sink): Fix data stream timestamp handling (#8271)

* chore(tests): Fix elasticsearch data streams test

This fixes the data streams test to ensure that `@timestamp` is set on
events are inserting into ES.

It also sets `break_events` to `false` to test that we actually insert
events into ES.

Signed-off-by: Jesse Szwedko <jesse@szwedko.me>

* Use the generated index when querying for data stream results

* Remove the added `data_stream` item before comparing test output

* Fix handling of timestamps in data stream mode

Signed-off-by: Bruce Guenter <bruce.guenter@datadoghq.com>

Co-authored-by: Jesse Szwedko <jesse@szwedko.me>