Port log4j-core-test migration to JUnit 5 into main

[AlbaHerrerias]

We are from Neighbourhoodie, the implementation partner of the STF Bug Resilience Program. This work is part of our agreed Milestone 1. Upgrade from JUnit 4 to JUnit 5. This PR takes the changes introduced in this PR and ports them into the main branch, as requested.

Most of the changes were not suitable:

• Many tests in main were already in JUnit 5
• The tests were not equal between 2.x and main (the tests in main need a separate and dedicated refactor)
• 2.x has test suites not present in main

Thank you!

Checklist

• Base your changes on 2.x branch if you are targeting Log4j 2; use main otherwise
• ./mvnw verify succeeds (if it fails due to code formatting issues reported by Spotless, simply run ./mvnw spotless:apply and retry)
• Non-trivial changes contain an entry file in the src/changelog/.2.x.x directory
• Tests for the changes are provided
• Commits are signed (optional, but highly recommended)

[github-actions]

Job	Requested goals	Build Tool Version	Build Outcome	Build Scan®
build-macos-latest	clean install	3.9.8	✅
build-ubuntu-latest	clean install	3.9.8	✅
build-windows-latest	clean install	3.9.8	✅

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