Support system tests on elastic_agent package

The `elastic_agent` package is a special case. It contains data streams, but these data streams don't contain any agent configuration. These data streams are intended to store the data sent by Elastic Agent.

The main difference with other packages is that the policy created doesn't need to have any package policy, but it needs to enable self-monitoring, what makes the agent start sending data to the installed datastreams.
Then the test should proceed as usual, checking the data stream and the documents stored.

Start by locating the existing system tests for other packages and the code that creates their policies and checks data streams. Compare that flow with the elastic_agent package: its policy should have no package policy but must enable self-monitoring. Done means the test runs through the usual data-stream and stored-document checks for elastic_agent.

Written by the indexing model from the issue text.