[Security Solution] data_source field diff algorithm test plan (#18…

…9669)

## Summary

Related ticket: #187659

Adds test plan for diff algorithm for arrays of scalar values
implemented here: #188874



### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)