[HDFS-10498] Intermittent test failure TestSnapshotFileLength.testSnapshotFileLength. Contributed by Jim Brennan.

(cherry picked from commit 6184ab07f670c181d1fda99e76729e920fff287e)

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/snapshot/TestSnapshotFileLength.java

@@ -128,6 +128,7 @@ public class TestSnapshotFileLength {
         hdfs.getFileChecksum(file1snap1), is(snapChksum1));
     try {
       AppendTestUtil.write(out, 0, toAppend);
+      out.hflush();
       // Test reading from snapshot of file that is open for append
       byte[] dataFromSnapshot = DFSTestUtil.readFileBuffer(hdfs, file1snap1);
       assertThat("Wrong data size in snapshot.",