How can I run kotlintest tests with gradle?

The kotlintest tests run perfectly fine when started from Intellij, but when I try to run them with the gradle test task command, only my regular JUnit tests are found and run.

The kotlintest code:

import io.kotlintest.matchers.shouldBe
import io.kotlintest.specs.StringSpec

class HelloKotlinTest : StringSpec() {
    init {
        println("Start Kotlin UnitTest")

        "length should return size of string" {
            "hello".length shouldBe 5
        }
    }
}

build.gradle:

apply plugin: 'org.junit.platform.gradle.plugin'

buildscript {
    ext.kotlinVersion = '1.1.3'
    ext.junitPlatformVersion = '1.0.0-M4'

    repositories {
        maven { url 'http://nexus.acompany.ch/content/groups/public' }
    }
    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"
        classpath "org.junit.platform:junit-platform-gradle-plugin:$junitPlatformVersion"
    }
}

sourceSets {
    main.kotlin.srcDirs += 'src/main/kotlin'
    test.kotlin.srcDirs += 'test/main/kotlin'
}

(...) 

dependencies {
    // Kotlin
    compile group: 'org.jetbrains.kotlin', name: 'kotlin-stdlib-jre8', version: kotlinVersion

    // Kotlin Test
    testCompile group: 'io.kotlintest', name: 'kotlintest', version: kotlinTestVersion

    // JUnit 5
    testCompile group: 'org.junit.jupiter', name: 'junit-jupiter-api', version: junitJupiterVersion
    testRuntime group: 'org.junit.jupiter', name: 'junit-jupiter-engine', version: junitJupiterVersion
}

In KotlinTest 3.1.x you don't need to use Junit4 anymore. It is fully compatible with JUnit 5. Thus the answer to your question is to upgrade to the 3.1.x track.

You need to add useJUnitPlatform() to the test block in your build.gradle.

You need to add testCompile 'io.kotlintest:kotlintest-runner-junit5:3.1.9' to your dependencies.

For example.

dependencies {
    testCompile 'io.kotlintest:kotlintest-runner-junit5:3.1.9'
}

test {
    useJUnitPlatform()

    // show standard out and standard error of the test JVM(s) on the console
    testLogging.showStandardStreams = true

    testLogging {
        events "PASSED", "FAILED", "SKIPPED", "STANDARD_OUT", "STANDARD_ERROR"
    }
}