configure<JUnitPlatformExtension> {
    engines {
        include("spek")
    }
}

// extension for configuration
fun JUnitPlatformExtension.engines(setup: EnginesExtension.() -> Unit) {
    when (this) {
        is ExtensionAware -> extensions.getByType(EnginesExtension::class.java).setup()
        else -> throw Exception("${this::class} must be an instance of ExtensionAware")
    }
}

> Extension of type 'EnginesExtension' does not exist. Currently registered extension types: [DefaultExtraPropertiesExtension, SelectorsExtension_Decorated, FiltersExtension_Decorated]

configure<JUnitPlatformExtension> {
    filters {
        engines { … }
    }
}

fun JUnitPlatformExtension.filters(setup: FiltersExtension.() -> Unit) {
    when (this) {
        is ExtensionAware -> extensions.getByType(FiltersExtension::class.java).setup()
        else -> throw Exception("${this::class} must be an instance of ExtensionAware")
    }
}

fun FiltersExtension.engines(setup: EnginesExtension.() -> Unit) {
    when (this) {
        is ExtensionAware -> extensions.getByType(EnginesExtension::class.java).setup()
        else -> throw Exception("${this::class} must be an instance of ExtensionAware")
    }
}

1.0.0-M3

configure<JUnitPlatformExtension>

Warning:Conflict with dependency 'org.jetbrains.kotlin:kotlin-stdlib'. Resolved versions for app (1.0.6-eap-112) and test app (1.0.4) differ. See <http://g.co/androidstudio/app-test-app-conflict> for details.