[SPARK-5937][YARN] Fix ClientSuite to set YARN mode, so that the correct class is used in t...

...ests.

Without this SparkHadoopUtil is used by the Client instead of YarnSparkHadoopUtil.

Author: Hari Shreedharan <hshreedharan@apache.org>

Closes #4711 from harishreedharan/SPARK-5937 and squashes the following commits:

d154de6 [Hari Shreedharan] Use System.clearProperty() instead of setting the value of SPARK_YARN_MODE to empty string.
f729f70 [Hari Shreedharan] Fix ClientSuite to set YARN mode, so that the correct class is used in tests.

yarn/src/test/scala/org/apache/spark/deploy/yarn/ClientSuite.scala

@@ -28,8 +28,7 @@ import org.apache.hadoop.yarn.api.records._
 import org.apache.hadoop.yarn.conf.YarnConfiguration
 import org.mockito.Matchers._
 import org.mockito.Mockito._
-import org.scalatest.FunSuite
-import org.scalatest.Matchers
+import org.scalatest.{BeforeAndAfterAll, FunSuite, Matchers}
 
 import scala.collection.JavaConversions._
 import scala.collection.mutable.{ HashMap => MutableHashMap }
@@ -39,7 +38,15 @@ import scala.util.Try
 import org.apache.spark.{SparkException, SparkConf}
 import org.apache.spark.util.Utils
 
-class ClientSuite extends FunSuite with Matchers {
+class ClientSuite extends FunSuite with Matchers with BeforeAndAfterAll {
+
+  override def beforeAll(): Unit = {
+    System.setProperty("SPARK_YARN_MODE", "true")
+  }
+
+  override def afterAll(): Unit = {
+    System.clearProperty("SPARK_YARN_MODE")
+  }
 
   test("default Yarn application classpath") {
     Client.getDefaultYarnApplicationClasspath should be(Some(Fixtures.knownDefYarnAppCP))