import org.gradle.api.plugins.ExtensionAware

import org.junit.platform.gradle.plugin.EnginesExtension
import org.junit.platform.gradle.plugin.JUnitPlatformExtension

// ...
// apply plugin, etc..
// ...

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

fun JUnitPlatformExtension.engines(setup: EnginesExtension.() -> Unit) {
    if(this is ExtensionAware){
        extensions.getByType(EnginesExtension::class.java).setup()
    }
}