Gradle TestKit compatibility

Extensions to test Gradle plugins with Prepared. Goal:

suite("Test a Gradle plugin") {
    test("This is a test") {
        Gradle.settingsKts("…")
        Gradle.rootBuildKts("…")
        
        Gradle.runner()
            .withArguments("build")
            .build()
    }
}

Doc: https://docs.gradle.org/current/userguide/testing_gradle_plugins.html