HDFS-13131. Modifying testcase testEnableAndDisableErasureCodingPolicy. Contributed by chencan.

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

@@ -1926,10 +1926,12 @@ public class TestDistributedFileSystem {
       fs.addErasureCodingPolicies(policies);
       assertEquals(policyName, ErasureCodingPolicyManager.getInstance().
           getByName(policyName).getName());
-      fs.disableErasureCodingPolicy(policyName);
       fs.enableErasureCodingPolicy(policyName);
       assertEquals(policyName, ErasureCodingPolicyManager.getInstance().
-          getByName(policyName).getName());
+          getEnabledPolicyByName(policyName).getName());
+      fs.disableErasureCodingPolicy(policyName);
+      assertNull(ErasureCodingPolicyManager.getInstance().
+          getEnabledPolicyByName(policyName));
 
       //test enable a policy that doesn't exist
       try {