Found https://docs.gradle.org/nightly/userguide/test_kit.html
Just to be sure I understand the use case for this kit:
It will allow build writers /plug-in writers to test their logic.
It’s the public facet to expose a clean API to perform Gradle build integration tests, much like you actually do for the integration tests of the gradle distribution.
Do I miss something?
Yes, exactly. The idea is to provide a standard mechanism for testing build logic, custom tasks, plugins, etc. This is something folks have been asking about for awhile and something that makes sense to exist in Gradle core rather than relying on third party tools like nebula-test. If you’re curious to know more detail, the design spec is also available for you to take a look at.
https://github.com/gradle/gradle/blob/master/design-docs/testing-toolkit.md