Update compile-testing dependency to 0.21.0

pf4j/pom.xml

@@ -170,7 +170,7 @@
         <dependency>
             <groupId>com.google.testing.compile</groupId>
             <artifactId>compile-testing</artifactId>
-            <version>0.18</version>
+            <version>0.21.0</version>
             <scope>test</scope>
         </dependency>
     </dependencies>