[Logs UI] [Alerting] Expand executor tests

[Kerry350]

Following on from the work in #68250 the logs alerts executor tests now only cover the ungrouped scenario, and not grouped scenarios.

This ticket should cover two sets of changes:

• The grouped scenario should be tested.

• The executor unit tests should be simplified in their approach, this wasn't as relevant before, but it is now there's two forks of logic. Currently the unit tests are heavily mocked all the way through to the callCluster method. We can simplify these tests by calling and testing smaller functions separately. Rather than trying to mimic an end-to-end flow via unit tests.

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)

[jasonrhodes]

+1 on getting this in sooner than later