HDFS-15702. Fix intermittent falilure of TestDecommission#testAllocAndIBRWhileDecommission. (#2507)

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestDecommission.java

@@ -976,6 +976,8 @@ public class TestDecommission extends AdminStatesBaseTest {
   @Test(timeout=120000)
   public void testAllocAndIBRWhileDecommission() throws IOException {
     LOG.info("Starting test testAllocAndIBRWhileDecommission");
+    getConf().setLong(DFSConfigKeys.DFS_BLOCKREPORT_INTERVAL_MSEC_KEY,
+        DFSConfigKeys.DFS_BLOCKREPORT_INTERVAL_MSEC_DEFAULT);
     startCluster(1, 6);
     getCluster().waitActive();
     FSNamesystem ns = getCluster().getNamesystem(0);