Add support for additional fields at `message` level in pipeline tests

Ex:

test-datastream-raw.log file contains message value:
`{ valid json}`

Pipeline tests are successful

if the file has the following format:

`{'some_field': 'hello', 'message':'{valid json}'}` then pipeline tests consider the entire log part of the `message`

Ex https://beats-ci.elastic.co/blue/organizations/jenkins/Ingest-manager%2Fintegrations/detail/PR-1462/8/tests
where the external fields like `azure_log_forwarder` were placed inside message