How do I enable "--enable-preview" for tests?

How do I enable "--enable-preview" for tests in Kotlin-based Gradle script ? I tried literally everything I could find online with https://stackoverflow.com/a/61849770/226895 being the closest to correct answer.

I still get following error on :test task

org.gradle.api.internal.tasks.testing.TestSuiteExecutionException: Could not execute test class 'com.blabla.playground.AppTest'.
    at org.gradle.api.internal.tasks.testing.SuiteTestClassProcessor.processTestClass(SuiteTestClassProcessor.java:53)
Caused by: java.lang.UnsupportedClassVersionError: Preview features are not enabled for com/blabla/playground/AppTest (class file version 58.65535). Try running with '--enable-preview'
    at java.base/java.lang.ClassLoader.defineClass1(Native Method)

by script is

plugins {
    java
    application
}

repositories {
    jcenter()
}

dependencies {
    implementation("com.google.guava:guava:28.2-jre")
    testImplementation("org.junit.jupiter:junit-jupiter-api:5.6.0")
    testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine:5.6.0")
}

application {
    mainClassName = "com.blabla.playground.App"
}

java {
    sourceCompatibility = JavaVersion.VERSION_14
    targetCompatibility = JavaVersion.VERSION_14
}

tasks {
    withType<Test>().all {
        allJvmArgs.add("--enable-preview")
        testLogging.showStandardStreams = true
        testLogging.showExceptions = true
        useJUnitPlatform {
        }
    }

    withType<JavaExec>().all {
        jvmArgs!!.add("--enable-preview")
    }

    withType<Wrapper>().all {
        gradleVersion = "6.4.1"
        distributionType = Wrapper.DistributionType.BIN
    }

    withType(JavaCompile::class.java).all {
        options.compilerArgs.addAll(listOf("--enable-preview", "-Xlint:preview"))
    }

    jar {
        manifest {
            attributes("Main-Class" to "com.blabla.playground.App")
        }
    }
}

What am I missing?

It turned out that the way you are configuring the flag for Test tasks does not really append the flag to allJvmArgs.

I managed to made it work by configuring the Test tasks like so :

withType<Test>().all {
    jvmArgs("--enable-preview")
}

or

withType<Test>().all {
    allJvmArgs = listOf("--enable-preview")
}

However the second option might not be preferable as it substitutes all JVM arguments for forked process (including arguments to define system properties, the minimum/maximum heap size, and the bootstrap classpath). The first option should be preferrable.