title: Unresolved reference: SpringExtension using @ExtendWith annotation
body: <p>I'm using trying to use Junit 5 within SpringBoot in order to take advantage of a number of new features that it provides. The basic setup works and I can use the new annotations such as <code>@BeforeAll</code> and <code>@AfterAll</code> however, I believe that because I cannot resolve the <code>@ExtendWith(SpringExtension::class)</code> I am not able to initialise my Test class to make use of injected resources. I have tried adding required dependencies to my <code>build.gradle</code> file but I assume I'm missing a key dependency:</p>
  
  <pre><code>buildscript {
  
  ext.dokka_version = '0.9.15'
  ext.dockerVersion = '1.2'
  ext.junit4Version = '4.12'
  ext.junitVintageVersion = '4.12.2'
  ext.junitPlatformVersion = '1.0.2'
  ext.junitJupiterVersion = '5.0.2'
  ext.kotlin_version = '1.1.60'
  ext.log4jVersion = '2.9.0'
  ext.springVersion = '1.5.8.RELEASE'
  
  repositories {
      mavenCentral()
      jcenter()
  }
  
  dependencies {
      // Needed for the 'kotlin' plugin
      classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:${kotlin_version}")
      // Needed for the 'org.springframework.boot' plugin
      classpath("org.springframework.boot:spring-boot-gradle-plugin:${springVersion}")
      // Needed for the 'kotlin-spring' plugin
      classpath("org.jetbrains.kotlin:kotlin-allopen:${kotlin_version}")
      //Needed for 'docker' plugin
      classpath("se.transmode.gradle:gradle-docker:${dockerVersion}")
      //Needed for 'dokka' plugin
      classpath("org.jetbrains.dokka:dokka-gradle-plugin:${dokka_version}")
      // Needed for Junit 5
      classpath("org.junit.platform:junit-platform-gradle-plugin:${junitPlatformVersion}")
  }
  
  }
  
  // Allows us to compile Kotlin files
  apply plugin: 'kotlin'
  // Does some extra work to set up Spring Boot.
  // Specifically this gives us the "bootRun" task we will be using
  apply plugin: 'org.springframework.boot'
  // Allows for improved interoperability between Kotlin and Spring
  apply plugin: 'kotlin-spring'
  // Allows us to create docker image
  apply plugin: 'docker'
  //Allows us to create API documentation
  apply plugin: 'org.jetbrains.dokka'
  
  apply plugin: 'idea'
  
  apply plugin: 'java'
  
  apply plugin: 'org.junit.platform.gradle.plugin'
  
  
  junitPlatform {
      // platformVersion '1.0.2'
      filters {
          engines {
              // include 'junit-jupiter', 'junit-vintage'
              // exclude 'custom-engine'
          }
          tags {
              // include 'fast'
              exclude 'slow'
          }
          // includeClassNamePattern '.*Test'
      }
      // enableStandardTestTask true
      // reportsDir file('build/test-results/junit-platform') // this is the default
      //logManager 'org.apache.logging.log4j.jul.LogManager'
  }
  
  task buildDocker(type: Docker, dependsOn: [build, dokka]) {
      applicationName = jar.baseName
      dockerfile = file('Dockerfile')
      doFirst {
          copy {
              from jar
              into stageDir
          }
      }
  }
  
  dokka {
      outputFormat = 'html'
      outputDirectory = $buildDir / doc
  }
  
  repositories {
      mavenCentral()
  }
  
  compileJava {
      sourceCompatibility = 1.8
      targetCompatibility = 1.8
  }
  
  compileTestJava {
      sourceCompatibility = 1.8
      targetCompatibility = 1.8
      options.compilerArgs += '-parameters'
  }
  
  dependencies {
  // JPA and H2
  compile("org.springframework.boot:spring-boot-starter-data-jpa")
  compile("com.h2database:h2")
  
  compile "org.hibernate:hibernate-entitymanager:5.2.10.Final"
  compile "org.hibernate:hibernate-core:5.2.10.Final"
  
  compile "org.jetbrains.kotlin:kotlin-stdlib-jre8:$kotlin_version"
  
  compile "org.slf4j:slf4j-api:1.7.25"
  
  // Kotlin Dependencies
  compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
  compile "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"
  testCompile "org.jetbrains.kotlin:kotlin-test-junit:$kotlin_version"
  
  testCompile "org.springframework.boot:spring-boot-starter-test"
  //testCompile "org.springframework.boot:spring-boot-starter-test" {
  //    exclude(module = "junit")
  //}
  // Mockito
  //testCompile "org.mockito:mockito-core:2.11.0"
  //testCompile "com.nhaarman:mockito-kotlin-kt1.1:1.5.0"
  
  //AssertJ
  testCompile 'org.assertj:assertj-core:3.8.0'
  
  // JUnit Jupiter API and TestEngine implementation
  testCompile("org.junit.jupiter:junit-jupiter-api:${junitJupiterVersion}")
  testCompile("org.junit.platform:juint-platform-runner:${junitPlatformVersion}")
  testRuntime("org.junit.jupiter:junit-jupiter-engine:${junitJupiterVersion}")
  
  // Only needed to run tests in an (IntelliJ) IDE(A) that bundles an older version
  testRuntime("org.junit.platform:junit-platform-launcher:${junitPlatformVersion}")
  
  // If you also want to support JUnit 3 and JUnit 4 tests
  //testCompile ("junit:junit:${junit4Version}")
  //testRuntime ("org.junit.vintage:junit-vintage-engine:${junitVintageVersion}")
  
  // To avoid compiler warnings about @API annotations in JUnit code
  testCompileOnly("org.apiguardian:apiguardian-api:1.0.0")
  
  // Spring Dependencies
  compile("org.springframework.boot:spring-boot-starter-web") {
      exclude module: "spring-boot-starter-tomcat"
  }
  compile "org.springframework.boot:spring-boot-starter-jetty"
  compile "org.springframework.boot:spring-boot-starter-actuator"
  
  // Jackson Dependencies
  compile "com.fasterxml.jackson.core:jackson-annotations"
  compile "com.fasterxml.jackson.core:jackson-core"
  compile "com.fasterxml.jackson.core:jackson-databind"
  runtime "com.fasterxml.jackson.datatype:jackson-datatype-jdk8"
  runtime "com.fasterxml.jackson.datatype:jackson-datatype-jsr310"
  runtime "com.fasterxml.jackson.module:jackson-module-kotlin"
  }
  
  configurations.all {
      exclude group: "org.slf4j", module: "slf4j-log4j12"
      exclude group: "log4j", module: "log4j"
  }
  
  task wrapper(type: Wrapper) {
      gradleVersion = "4.3"
  }
  </code></pre>