[alerting event log] add more tests for existing but unused code

[pmuellr]

The code in the event log that deals with writing to ES is currently shipping as disabled by default, and to get the plugin merged to master sooner, writing tests for that code was deferred.

Time to add it! This is primarily code in the https://github.com/elastic/kibana/tree/master/x-pack/plugins/event_log/server/es directory, and it needs both jest and functional tests.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[mikecote]

Some unit tests were added with the migrations issue (#58010). This issue can focus on adding functional / integration tests for indexing into Elasticsearch.