HADOOP-12701. Run checkstyle on test source files. Contributed by John Zhuge.

(cherry picked from commit e097c9a124dc0b9ae9076994d19663f29d771ef0)

pom.xml

@@ -208,6 +208,7 @@ xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xs
           </dependencies>
           <configuration>
             <configLocation>checkstyle/checkstyle.xml</configLocation>
+            <includeTestSourceDirectory>true</includeTestSourceDirectory>
             <failOnViolation>false</failOnViolation>
             <format>xml</format>
             <format>html</format>