How to use JUnit 5 with build.gradle.kts and kotlin?

There's documentation and a sample project for using Gradle and JUnit 5.

If you want to use the Kotlin DSL you'll have to adapt it in the following way:

tasks.withType<Test>().configureEach {
    useJUnitPlatform()
}

For Jigsaw, Gradle provides experimental support and there's a fork of the plugin that provides additional features.

Marc Philipp's answer works. Here is another approach:

val test: Test by tasks
test.useJUnitPlatform()