YARN-4916. TestNMProxy.tesNMProxyRPCRetry fails. Contributed by Tibor Kiss.

(cherry picked from commit 00058167431475c6e63c80207424f1d365569e3a)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-nodemanager/src/test/java/org/apache/hadoop/yarn/server/nodemanager/containermanager/TestNMProxy.java

@@ -188,8 +188,7 @@ public class TestNMProxy extends BaseContainerManagerTest {
       Assert.fail("should get socket exception");
     } catch (IOException e) {
       // socket exception should be thrown immediately, without RPC retries.
-      Assert.assertTrue(e.toString().
-          contains("Failed on local exception: java.net.SocketException"));
+      Assert.assertTrue(e instanceof java.net.SocketException);
     }
   }