Adding test source scanning to Checkstyle config

Backport of 4cef85d from the main jclouds repo
diff --git a/project/pom.xml b/project/pom.xml
index e0d70ff..bf02349 100644
--- a/project/pom.xml
+++ b/project/pom.xml
@@ -307,6 +307,7 @@
         <configuration>
           <configLocation>resources/checkstyle.xml</configLocation>
           <violationSeverity>warning</violationSeverity>
+          <includeTestSourceDirectory>true</includeTestSourceDirectory>
         </configuration>
       </plugin>
     </plugins>