Migrate log4j-slf4j-impl to JUnit 5

[AlbaHerrerias]

Hello 👋

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 migrates the tests located in log4j-slf4j-impl.

This refactor is related to #3080, where I was asked to replace some assertions to use assertInstanceOf instead.

Please note that, with the removal of junit-vintage-engine, the tests of Log4j2_1482_Slf4jTest.java are not being executed (total of 2 tests). They extend from log4j-core-test/.../Log4j2_1482_Test.java, currently written in JUnit 4. My coworker is refactoring those in #3061.

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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[ppkarwasz]

LGTM, thanks!