[System Actions] Configure integration tests for alerting plugin endpoints for sysactions

[Zacqary]

As part of the System Actions initiative, we're updating many APIs to output different action responses depending on if something is a system action or not. We do not have any E2E tests to validate that these outputs are correct.

We may not be able to write these tests until we've actually added at least one system action.

This issue serves as a discussion point for which endpoints need to have integration tests written or updated to deal with system actions, or any new tests we have to write to cover system actions.

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)