[HOTFIX] Do not throw NPE if spark.test.home is not set

`spark.test.home` was introduced in #1734. This is fine for SBT but is failing maven tests. Either way it shouldn't throw an NPE.

Author: Andrew Or <andrewor14@gmail.com>

Closes #1739 from andrewor14/fix-spark-test-home and squashes the following commits:

ce2624c [Andrew Or] Do not throw NPE if spark.test.home is not set

core/src/main/scala/org/apache/spark/deploy/worker/Worker.scala

@@ -71,7 +71,7 @@ private[spark] class Worker(
   // TTL for app folders/data;  after TTL expires it will be cleaned up
   val APP_DATA_RETENTION_SECS = conf.getLong("spark.worker.cleanup.appDataTtl", 7 * 24 * 3600)
 
-
+  val testing: Boolean = sys.props.contains("spark.testing")
   val masterLock: Object = new Object()
   var master: ActorSelection = null
   var masterAddress: Address = null
@@ -82,7 +82,12 @@ private[spark] class Worker(
   @volatile var connected = false
   val workerId = generateWorkerId()
   val sparkHome =
-    new File(sys.props.get("spark.test.home").orElse(sys.env.get("SPARK_HOME")).getOrElse("."))
+    if (testing) {
+      assert(sys.props.contains("spark.test.home"), "spark.test.home is not set!")
+      new File(sys.props("spark.test.home"))
+    } else {
+      new File(sys.env.get("SPARK_HOME").getOrElse("."))
+    }
   var workDir: File = null
   val executors = new HashMap[String, ExecutorRunner]
   val finishedExecutors = new HashMap[String, ExecutorRunner]

core/src/test/scala/org/apache/spark/DriverSuite.scala

@@ -34,7 +34,7 @@ import scala.language.postfixOps
 class DriverSuite extends FunSuite with Timeouts {
 
   test("driver should exit after finishing") {
-    val sparkHome = sys.props("spark.test.home")
+    val sparkHome = sys.props.getOrElse("spark.test.home", fail("spark.test.home is not set!"))
     // Regression test for SPARK-530: "Spark driver process doesn't exit after finishing"
     val masters = Table(("master"), ("local"), ("local-cluster[2,1,512]"))
     forAll(masters) { (master: String) =>

core/src/test/scala/org/apache/spark/deploy/SparkSubmitSuite.scala

@@ -295,7 +295,7 @@ class SparkSubmitSuite extends FunSuite with Matchers {
 
   // NOTE: This is an expensive operation in terms of time (10 seconds+). Use sparingly.
   def runSparkSubmit(args: Seq[String]): String = {
-    val sparkHome = sys.props("spark.test.home")
+    val sparkHome = sys.props.getOrElse("spark.test.home", fail("spark.test.home is not set!"))
     Utils.executeAndGetOutput(
       Seq("./bin/spark-submit") ++ args,
       new File(sparkHome),

core/src/test/scala/org/apache/spark/deploy/worker/ExecutorRunnerTest.scala

@@ -27,7 +27,7 @@ import org.apache.spark.SparkConf
 class ExecutorRunnerTest extends FunSuite {
   test("command includes appId") {
     def f(s:String) = new File(s)
-    val sparkHome = sys.props("spark.test.home")
+    val sparkHome = sys.props.getOrElse("spark.test.home", fail("spark.test.home is not set!"))
     val appDesc = new ApplicationDescription("app name", Some(8), 500,
       Command("foo", Seq(), Map(), Seq(), Seq(), Seq()), "appUiUrl")
     val appId = "12345-worker321-9876"

pom.xml

@@ -868,10 +868,10 @@
             <filereports>${project.build.directory}/SparkTestSuite.txt</filereports>
             <argLine>-Xmx3g -XX:MaxPermSize=${MaxPermGen} -XX:ReservedCodeCacheSize=512m</argLine>
             <stderr/>
-            <environmentVariables>
-              <SPARK_HOME>${session.executionRootDirectory}</SPARK_HOME>
-              <SPARK_TESTING>1</SPARK_TESTING>
-            </environmentVariables>
+            <systemProperties>
+              <spark.test.home>${session.executionRootDirectory}</spark.test.home>
+              <spark.testing>1</spark.testing>
+            </systemProperties>
           </configuration>
           <executions>
             <execution>