[CALCITE-6048] ServerTest#testTruncateTable fails intermittently due …

…to method not found exception

Close apache#3567

core/src/main/java/org/apache/calcite/util/ReflectUtil.java

@@ -436,7 +436,7 @@ E extends Object> ReflectiveVisitDispatcher<R, E> createDispatcher(
             Collections.emptyList());
       }
 
-      @Override public @Nullable Method lookupVisitMethod(
+      @Override public synchronized @Nullable Method lookupVisitMethod(
           Class<? extends R> visitorClass,
           Class<? extends E> visiteeClass,
           String visitMethodName,