title: @SpringBootTest annotation on interfrace not working
body: <p>It is not possible right now to place the annotation <code>SpringBootTest</code> on an interface, inherit from the interface and run the test.</p>
  
  <pre><code>@ExtendWith(SpringExtension::class)
  @SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
  interface SpringBootTestBase
  
  class GreetingControllerITest : SpringBootTestBase {
  
      @Autowired
      private lateinit var restTemplate: TestRestTemplate
  
      @Test
      fun `spring boot endpoint gets correct greeting`() {
          val body = restTemplate.getForObject("/greet/World", String::class.java)
          assertThat(body).isEqualTo("Hello, World!")
      }
  }
  </code></pre>
  
  <p>This fails with a NPE</p>
  
  <pre><code>Caused by: java.lang.NullPointerException: null
      at org.springframework.boot.test.context.SpringBootTestContextCustomizer.customizeContext(SpringBootTestContextCustomizer.java:50) ~[spring-boot-test-2.0.0.M6.jar:2.0.0.M6]
      at org.springframework.boot.test.context.SpringBootContextLoader$ContextCustomizerAdapter.initialize(SpringBootContextLoader.java:326) ~[spring-boot-test-2.0.0.M6.jar:2.0.0.M6]
      at org.springframework.boot.SpringApplication.applyInitializers(SpringApplication.java:625) ~[spring-boot-2.0.0.M6.jar:2.0.0.M6]
      at org.springframework.boot.SpringApplication.prepareContext(SpringApplication.java:365) ~[spring-boot-2.0.0.M6.jar:2.0.0.M6]
      at org.springframework.boot.SpringApplication.run(SpringApplication.java:325) ~[spring-boot-2.0.0.M6.jar:2.0.0.M6]
      at org.springframework.boot.test.context.SpringBootContextLoader.loadContext(SpringBootContextLoader.java:138) ~[spring-boot-test-2.0.0.M6.jar:2.0.0.M6]
      at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContextInternal(DefaultCacheAwareContextLoaderDelegate.java:99) ~[spring-test-5.0.1.RELEASE.jar:5.0.1.RELEASE]
      at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:117) ~[spring-test-5.0.1.RELEASE.jar:5.0.1.RELEASE]
      ... 61 common frames omitted
  </code></pre>
  
  <p>The reason for this interface is, to put all annotations on the Interface and let all our tests inherit from that one -> avoid annotation-code-duplication</p>
  
  <p>Using a base-class instead an interface is no real option for me, because I have other base-classes to inherit from, which are not test-related.</p>
  
  <p>Would this be a useful feature I should bring to the spring community?</p>