[MINOR] improvement(mr): Add @test to activate test case (#923)

### What changes were proposed in this pull request?
Add @test to activate test case

### Why are the changes needed?

Just for Add @test to activate test case

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
unit test

client-mr/core/src/test/java/org/apache/hadoop/mapreduce/task/reduce/FetcherTest.java

@@ -192,6 +192,7 @@ public void writeAndReadDataMergeFailsTestWithRss() throws Throwable {
     assertEquals(2, ((MockMergeManagerImpl)merger).happenedFails.size());
   }
 
+  @Test
   public void testCodecIsDuplicated() throws Exception {
     fs = FileSystem.getLocal(conf);
     BZip2Codec codec = new BZip2Codec();