title: Powermock throws ClassNotPreparedException when using JUnit 5
body: <p>I have a sample test class where I want to mock a static class.My build.gradle is like  </p>
  
  <pre><code>testCompile group: 'org.junit.jupiter', name: 'junit-jupiter-api', version: '5.2.0'
  testCompile group: 'org.junit.jupiter', name: 'junit-jupiter-params', version: '5.2.0'
  testRuntime group: 'org.junit.jupiter', name: 'junit-jupiter-engine', version: '5.2.0'
  testRuntime("org.junit.platform:junit-platform-launcher:1.1.1")
  
  testCompile group: 'org.powermock', name: 'powermock-api-mockito2', version: '2.0.0-beta.5'
  testCompile group: 'org.mockito', name: 'mockito-core', version: '2.21.0'
  testCompile group: 'org.mockito', name: 'mockito-junit-jupiter', version: '2.21.0'`
  </code></pre>
  
  <p>When I try my test case as </p>
  
  <pre><code>@ExtendWith(MockitoExtension.class)
  @PrepareForTest(MyUtil.class)
  public class MyTest {
  
  @Test
  public void shouldCheckIfSyncTimeIsAboveThreshold() {
      mockStatic(MyUtil.class);
      when(MyUtil.getValue()).thenReturn(5));
   }
  </code></pre>
  
  <p>But when I run this, I got exception like </p>
  
  <pre><code>org.powermock.api.mockito.ClassNotPreparedException:
  The class MyUtil not prepared for test
  </code></pre>
  
  <p>Is there any way that I can achieve the same</p>