How do I configure KoTest with Gradle in a LibGdx project? Currently can't build because StringSpec is not recognised

Question

I have a standard LibGdx project setup by the LibGdx tool, only targeting desktop. It uses Gradle (Groovy DSL) to manage dependencies and tasks. I've converted the core module to Kotlin, and I'm trying to add a Kotlin test module using Kotest.

I've followed the Kotest instructions for Gradle on their GitHub but the compile is failing because StringSpec isn't reocgnised (Unresolved reference: StringSpec). I think LibGdx's default Gradle setup may be a little outdated or use older syntax/structure, and perhaps it's conflicting with Kotest's instructions intended for newer versions of Gradle?

For now I've removed any test and am just trying to get it to recognise StringSpec and compile. I've not even reached the stage of getting IntelliJ to recognise and run the tests. Here's what I have so far:

core.tests/tests/com/me/game/AcceptanceTests.kt

package com.jansky.myproject

class AcceptanceTests : StringSpec() {

}

core.tests/gradle.build

version '1.0'

sourceCompatibility = 1.7
[compileJava, compileTestJava]*.options*.encoding = 'UTF-8'

sourceSets.main.java.srcDirs = [ "tests/" ]

tasks.withType(Test) {
    useJUnitPlatform()
}

eclipse.project.name = appName + "-core.tests"

./build.gradle (ie the root buildfile)

buildscript {
    ext.kotlinVersion = '1.3.71'

    repositories {
        mavenLocal()
        mavenCentral()
        maven { url "https://plugins.gradle.org/m2/" }
        maven { url "https://oss.sonatype.org/content/repositories/snapshots/" }
        jcenter()
        google()
    }
    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"
    }
}

allprojects {
    apply plugin: "eclipse"

    version = '1.0'
    ext {
        appName = "game"
        gdxVersion = '1.9.10'
        roboVMVersion = '2.3.8'
        box2DLightsVersion = '1.4'
        ashleyVersion = '1.7.0'
        aiVersion = '1.8.0'
    }

    repositories {
        mavenLocal()
        mavenCentral()
        jcenter()
        google()
        maven { url "https://oss.sonatype.org/content/repositories/snapshots/" }
        maven { url "https://oss.sonatype.org/content/repositories/releases/" }
    }
}

project(":desktop") {
    apply plugin: "kotlin"


    dependencies {
        implementation project(":core")
        api "com.badlogicgames.gdx:gdx-backend-lwjgl:$gdxVersion"
        api "com.badlogicgames.gdx:gdx-platform:$gdxVersion:natives-desktop"
        compile "com.badlogicgames.ashley:ashley:1.7.3"
        compile group: 'io.github.libktx', name: 'ktx-ashley', version: '1.9.10-b4'
    }
}

project(":core") {
    apply plugin: "kotlin"

    dependencies {
        api "com.badlogicgames.gdx:gdx:$gdxVersion"
        compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlinVersion"
        compile "org.jetbrains.kotlin:kotlin-reflect:$kotlinVersion"
        compile "com.badlogicgames.ashley:ashley:1.7.3"
        compile group: 'io.github.libktx', name: 'ktx-ashley', version: '1.9.10-b4'
    }
}

project(":core.tests") {
    apply plugin: "kotlin"

    test {
        useJUnitPlatform()
    }

    dependencies {
        api "com.badlogicgames.gdx:gdx:$gdxVersion"
        compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlinVersion"
        compile "org.jetbrains.kotlin:kotlin-reflect:$kotlinVersion"
        compile "com.badlogicgames.ashley:ashley:1.7.3"
        compile group: 'io.github.libktx', name: 'ktx-ashley', version: '1.9.10-b4'
        implementation 'io.kotest:kotest-runner-junit5:4.0.2'
        implementation 'io.kotest:kotest-assertions-core:4.0.2'
    }
}

settings.gradle

include 'desktop', 'core', 'core.tests'

Gradle-wrapper.properties

#Sat Apr 04 15:53:20 BST 2020
distributionUrl=https\://services.gradle.org/distributions/gradle-5.4.1-all.zip
distributionBase=GRADLE_USER_HOME
distributionPath=wrapper/dists
zipStorePath=wrapper/dists
zipStoreBase=GRADLE_USER_HOME

I don't have much JVM experience, so I'm at a bit of a loss. Hopefully I've missed something that's obvious to someone that knows Gradle better. Any ideas?