Upgrade to testng 6.8.21

Note that this is the last version compatible with Java 6.  Changelog:

https://github.com/cbeust/testng/blob/master/CHANGES.txt

pom.xml

@@ -367,7 +367,7 @@
     <dependency>
       <groupId>org.testng</groupId>
       <artifactId>testng</artifactId>
-      <version>6.8.8</version>
+      <version>6.8.21</version>
       <scope>test</scope>
     </dependency>
   </dependencies>