text: Title: Alerting Task Runner is heavily coupled with its dependnecies
  Body: The AlertingTask Runner does [a lot of things](https://github.com/elastic/kibana/blob/master/x-pack/plugins/alerts/server/task_runner/task_runner.ts) and its tests are heavily coupled with its dependencies.
  
  Mocking out dependencies such as the `createExecutionHandler` function breaks many tests and, as you can see in the tests, there's a lot of setup and dependency mocking throughout.
  This makes it hard change the runner and hard to reliably test it.
  
  We should try to break it down into smaller units that are easier to test independently.