Fix TestDFSAdmin#testAllDatanodesReconfig fails

Author: lfxy

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/tools/TestDFSAdmin.java

@@ -1250,6 +1250,7 @@ public void testAllDatanodesReconfig()
         any(Configuration.class))).thenReturn(changes);
 
     int result = admin.startReconfiguration("datanode", "livenodes");
+    Thread.sleep(50);
     assertThat(result).isEqualTo(0);
     final List<String> outsForStartReconf = new ArrayList<>();
     final List<String> errsForStartReconf = new ArrayList<>();