chore: audit junit tests with logging assertions

We have a variety of ways of asserting that logs are emitted, and have run into situations where the order of tests being run affects whether assertions succeed. We should audit these methods and standardize the way we verify that logs are emitted so that we stop running into these frustrations.

Related:
- https://github.com/elastic/logstash/pull/16719#discussion_r1856937636
- https://github.com/elastic/logstash/pull/16643#discussion_r1911727880
- anything that uses [`logstash-core/src/test/resources/log4j2*`](https://github.com/elastic/logstash/tree/main/logstash-core/src/test/resources) to modify the logging configuration during test execution