[MNG-5761] Dependency management is not transitive.

o Updated to add a missing test dependency (used but undeclared) managed to
  non-transitive test scope transitively. The compilation error without
  that declaration in fact is expected behaviour. Don't let them fool you.

  I can build all plugins using 3.4.0-SNAPSHOT by upgrading the plugin tools
  manually on the command line like:

  mvn -DmavenPluginToolsVersion=3.5 test-compile



git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1776591 13f79535-47bb-0310-9956-ffa450edef68

pom.xml

@@ -101,6 +101,12 @@ under the License.
       <version>${mavenVersion}</version>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <version>4.12</version>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 
   <profiles>