Add testRuntimeOnly dependency on junit-platform-launcher with Gradle 8.+

[timtebeek]

What's changed?

• Add recipe to add Gradle dependency on org.junit.platform:junit-platform-launcher
• Add transitive dependency when using rewrite-test

What's your motivation?

Reduce the warnings seen in build logs when using Gradle 8.x, ahead of a migration to Gradle 9.
https://docs.gradle.org/8.11.1/userguide/upgrading_version_8.html#test_framework_implementation_dependencies

Anything in particular you'd like reviewers to focus on?

Would the api dependency already prevent that we have to add the testRuntimeOnly dependency for rewrite-*?

Have you considered any alternatives or workarounds?

We could delay this until Gradle 9, but the warnings are already annoying right now.

[shanman190]

Would the api dependency already prevent that we have to add the testRuntimeOnly dependency for rewrite-*?

Yeah, based on my understanding once rewrite-test is exposing the dependency in the api configuration, then all second-level modules will get the benefit of it being available. The one part about this that we will want to keep an eye on is if there version conflicts introduced due to the api dependency with the imported junit-platform-launcher in transitives which are depending upon how Gradle interacts with it at runtime.

On the flip side, the recommendation of placing the dependency into testRuntimeOnly means that it would be available for testing tasks, but would not be exported for others to use and consume.

[timtebeek]

You're very right to point out that the api("org.junit.platform:junit-platform-launcher") now comes in with the wrong scope, making it available to sources as opposed to runtime only. But without this change we'd have to update ~50+ build.gradle.kts files which for now only depend on rewrite-test, and expect the other transitive junit api dependencies already. I'd wanted to avoid those updates, especially since .kts isn't supported yet. With that I think we can tolerate the potential downsides, at least for now.