[fix](statistics)Fix AnalysisTaskExecutor unit test timeout. (#36456) (#36948)

Deencapsulation.getField sometime doesn't work as expected, which may
cause FE unit test timeout. Add get function in source code to make it
easier.
backport: https://github.com/apache/doris/pull/36456

fe/fe-core/src/main/java/org/apache/doris/statistics/AnalysisTaskExecutor.java

@@ -105,4 +105,9 @@ public class AnalysisTaskExecutor {
         executors.getQueue().clear();
         taskQueue.clear();
     }
+
+    // For unit test only.
+    public BlockingQueue<AnalysisTaskWrapper> getTaskQueue() {
+        return taskQueue;
+    }
 }