fix(java): add -ntp flag to native image testing command (#1299)

Author: dzou

synthtool/gcp/templates/java_library/.kokoro/build.sh

@@ -71,7 +71,7 @@ integration)
     ;;
 graalvm)
     # Run Unit and Integration Tests with Native Image
-    mvn test -Pnative -Penable-integration-tests
+    mvn -ntp -Pnative -Penable-integration-tests test
     RETURN_CODE=$?
     ;;
 samples)