Migrate tests to JUnit 6

[sissbruecker]

Currently all tests in flow-components use JUnit 4 APIs. However, Flow has started migrating tests to JUnit 6.

This has some side-effects:

• flow-test-generic declares dependencies for JUnit 6
• We use that in all Flow component modules to reuse ClassesSerializableTest
• JUnit 6 is being pulled into all Flow component modules
• Surefire plugin now runs tests with the JUnit 6 provider (org.apache.maven.surefire.junitplatform.JUnitPlatformProvider)

For now that seems to work fine, as the provider seems to automatically detect JUnit 4 and 6 being on the class path, and then picking a "vintage" engine that supports running both. However, this means our tests actually running depends on some heuristics in Surefire.

Short term we might want to configure Surefire so that, instead of relying auto-detection, we explicitly configure either:

• A JUnit 4 provider. Works as long as all tests use JUnit 4.
• Or configure the vintage engine explicitly. However not all modules currently pull in JUnit 6, so we might have to add explicit dependencies for that.

Long term we should migrate tests to JUnit 6:

• Research what differences are between versions and which API usages need to be updated
• Configure Surefire so that it supports running both JUnit 4 and JUnit 6 tests
• Gradually convert individual modules

Then do the same for integration tests, needs checking if there are any issues regarding TestBench.

[Artur-]

Just a side note: The Flow JUnit 6 conversion was 100% done by AI, in reviewable chunks

[sissbruecker]

Did you use some specific instructions on how to adapt tests to new patterns in JUnit 6?

[Artur-]

No

[knoobie]

Why not using OpenRewrite for it? https://docs.openrewrite.org/running-recipes/popular-recipe-guides/migrate-from-junit-4-to-junit-5 (there is no difference between 5 and 6)

[Artur-]

We wanted to do it in smaller pieces that can reasonably be reviewed. Thus the challenge was not really the conversion but to find independent sets to convert