[SPARK-6753] Clone SparkConf in ShuffleSuite tests

Prior to this change, the unit test for SPARK-3426 did not clone the
original SparkConf, which meant that that test did not use the options
set by suites that subclass ShuffleSuite.scala. This commit fixes that
problem.

Author: kayousterhout

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

@@ -242,14 +242,14 @@ abstract class ShuffleSuite extends FunSuite with Matchers with LocalSparkContex
       shuffleSpillCompress <- Set(true, false);
       shuffleCompress <- Set(true, false)
     ) {
-      val conf = new SparkConf()
+      val myConf = conf.clone()
         .setAppName("test")
         .setMaster("local")
         .set("spark.shuffle.spill.compress", shuffleSpillCompress.toString)
         .set("spark.shuffle.compress", shuffleCompress.toString)
         .set("spark.shuffle.memoryFraction", "0.001")
       resetSparkContext()
-      sc = new SparkContext(conf)
+      sc = new SparkContext(myConf)
       try {
         sc.parallelize(0 until 100000).map(i => (i / 4, i)).groupByKey().collect()
       } catch {