Gradle SourceSet dependencies in IntelliJ

Question

I've got a Gradle project with an additional sourceSet - acceptance. This contains my acceptance tests, as opposed to just build-time unit and integration tests.

I also have some helper classes in the standard test sourceSet that I'd like to share, but that don't belong in the main sourceSet.

Currently this works fine in Gradle, but IntelliJ doesn't like it and I'm not sure why.

My build.gradle looks like this:

buildscript {
  ext {
    jackson_version = "2.9.0.pr4"
    // IntelliJ needs M4
    junitJupiter_version = "5.0.0-M4"
    junitPlatform_version = "1.0.0-M4"
    kotlin_version = "1.1.3-2"
    slf4j_version = "1.7.25"
    spring_version = "4.3.10.RELEASE"
    springBoot_version = "1.5.4.RELEASE"
    springBootAdmin_version = "1.5.2"

    runAcceptance = System.properties['noAcceptance'] == null
  }

  repositories {
    mavenCentral()
    jcenter()
  }

  dependencies {
    classpath "com.github.ben-manes:gradle-versions-plugin:0.15.0"
    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    classpath "org.jetbrains.kotlin:kotlin-allopen:$kotlin_version"
    classpath "org.junit.platform:junit-platform-gradle-plugin:$junitPlatform_version"
    classpath "org.springframework.boot:spring-boot-gradle-plugin:$springBoot_version"
  }
}

plugins {
  id "com.github.ben-manes.versions" version "0.15.0"
}

apply plugin: "com.github.ben-manes.versions"
apply plugin: "kotlin"
apply plugin: "kotlin-spring"
apply plugin: "org.junit.platform.gradle.plugin"
apply plugin: "org.springframework.boot"
apply plugin: "war"

repositories {
  mavenCentral()
  maven { url "https://jitpack.io" }
}

sourceSets {
  acceptance {
    kotlin {
      compileClasspath += main.output + test.output
      runtimeClasspath += main.output + test.output
      srcDir file('src/acceptance/kotlin')
    }
    resources.srcDir file('src/acceptance/resources')
  }
}

configurations {
  acceptanceCompile.extendsFrom testCompile
  acceptanceRuntime.extendsFrom testRuntime
}

dependencies {
  compile "com.auth0:java-jwt:3.2.0"
  compile "com.graphql-java:graphql-java-tools:3.2.1"
  compile "com.graphql-java:graphql-spring-boot-starter:3.6.0"
  compile "com.zaxxer:HikariCP:2.6.3"
  compile("de.codecentric:spring-boot-admin-server:$springBootAdmin_version") {
      exclude group: "junit", module: "junit"
  }
  compile("de.codecentric:spring-boot-admin-server-ui:$springBootAdmin_version") {
      exclude group: "junit", module: "junit"
  }
  compile("de.codecentric:spring-boot-admin-starter-client:$springBootAdmin_version") {
      exclude group: "junit", module: "junit"
  }
  compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
  compile "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"
  compile "org.slf4j:slf4j-api:$slf4j_version"
  compile "org.springframework:spring-jdbc:$spring_version"
  compile "org.springframework.boot:spring-boot-starter-web:$springBoot_version"
  compile "org.springframework.boot:spring-boot-starter-actuator:$springBoot_version"
  compile "ru.yandex.qatools.embed:postgresql-embedded:2.2"

  runtime "ch.qos.logback:logback-classic:1.2.3"
  runtime "org.jolokia:jolokia-core:1.3.7"
  runtime "org.liquibase:liquibase-core:3.5.3"
  runtime "org.postgresql:postgresql:42.1.3"
  runtime "org.slf4j:jcl-over-slf4j:$slf4j_version"
  runtime "org.slf4j:jul-to-slf4j:$slf4j_version"

  testCompile "com.github.sbrannen:spring-test-junit5:1.0.0.M4"
  testCompile "com.nhaarman:mockito-kotlin:1.5.0"
  testCompile("org.jetbrains.kotlin:kotlin-test-junit:$kotlin_version") {
      exclude group: "junit", module: "junit"
  }
  testCompile "org.junit.jupiter:junit-jupiter-api:$junitJupiter_version"

  testRuntime "org.junit.jupiter:junit-jupiter-engine:$junitJupiter_version"

  acceptanceCompile "commons-jxpath:commons-jxpath:1.3"
  acceptanceCompile "org.springframework.boot:spring-boot-starter-test:$springBoot_version"
}

task acceptance(type: Test) {
  testClassesDirs = sourceSets.acceptance.output.classesDirs
  classpath = sourceSets.acceptance.runtimeClasspath
  outputs.upToDateWhen { false }
}

if (ext.runAcceptance) {
  check.dependsOn acceptance
}
acceptance.mustRunAfter test

task wrapper(type: Wrapper) {
  gradleVersion = "4.0"
}

IntelliJ can see dependencies from the main sourceSet in the acceptance one just fine, but refuses to allow me to access the test one - even though a command line build works. What am I missing?