Migrate log4j-iostreams to use JUnit 5 APIs and compatible helper classes

[jcoglan]

Hi, I'm 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-iostreams.

These changes are mostly straightforward replacements of the JUnit 4 annotations with JUnit 5 ones. The only thing I was unsure about is that I had to add constructors to all the test classes to accept the LoggerContext argument -- I was wondering if there's a way of just doing this in the abstract classes and having all their children just work without this extra boilerplate.

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)