title: Maven can't load groups for JUnit categories out of a class with multiple interfaces?
body: <p>My Maven:</p>
  
  <pre><code>            &lt;plugin&gt;
                  &lt;groupId&gt;org.apache.maven.plugins&lt;/groupId&gt;
                  &lt;artifactId&gt;maven-surefire-plugin&lt;/artifactId&gt;
                  &lt;version&gt;2.15&lt;/version&gt;
                  &lt;configuration&gt;
                      &lt;groups&gt;
                          com.rubberduck.TestState.Ready
                      &lt;/groups&gt;
                  &lt;/configuration&gt;
              &lt;/plugin&gt;
  </code></pre>
  
  <p>My Class:</p>
  
  <pre><code>package com.rubberduck;
  public class TestState
  {
      public interface Ready {
          /* to allow filter ready tests as groups */
      }    
      public interface InProgress {
          /* to allow filter ready tests as groups */
      }    
      public interface NotTested {
          /* to allow filter ready tests as groups */
      }    
  }
  </code></pre>
  
  <p>My Test:</p>
  
  <pre><code>@Test(timeout = 60000)
  @Category(TestState.Ready.class)
  public void test() throws Exception {
      assertEquals(true, true);
  }
  </code></pre>
  
  <p>My Error:</p>
  
  <pre><code>Failed to execute goal org.apache.maven.plugins:maven-surefire-plugin:2.14.1:test (default-cli) on project rubberduck: Execution default-cli of goal org.apache.maven.plugins:maven-surefire-plugin:2.14.1:test failed: There was an error in the forked process
  java.lang.RuntimeException: Unable to load category: com.rubberduck.TestState.Ready
  </code></pre>
  
  <p>If I give him <code>&lt;groups&gt;com.rubberduck.TestState&lt;/groups&gt;</code> it compiles without error, but I want to have multiple interfaces for groups in the same class, isn't that possible?</p>