[SPARK-14935][CORE] DistributedSuite "local-cluster format" shouldn't actually launch clusters

https://issues.apache.org/jira/browse/SPARK-14935

In DistributedSuite, the "local-cluster format" test actually launches a bunch of clusters, but this doesn't seem necessary for what should just be a unit test of a regex. We should clean up the code so that this is testable without actually launching a cluster, which should buy us about 20 seconds per build.

Passed unit test on my local machine

Author: Xin Ren <iamshrek@126.com>

Closes #12744 from keypointt/SPARK-14935.

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

@@ -51,18 +51,21 @@ class DistributedSuite extends SparkFunSuite with Matchers with LocalSparkContex
   }
 
   test("local-cluster format") {
-    sc = new SparkContext("local-cluster[2,1,1024]", "test")
-    assert(sc.parallelize(1 to 2, 2).count() == 2)
-    resetSparkContext()
-    sc = new SparkContext("local-cluster[2 , 1 , 1024]", "test")
-    assert(sc.parallelize(1 to 2, 2).count() == 2)
-    resetSparkContext()
-    sc = new SparkContext("local-cluster[2, 1, 1024]", "test")
-    assert(sc.parallelize(1 to 2, 2).count() == 2)
-    resetSparkContext()
-    sc = new SparkContext("local-cluster[ 2, 1, 1024 ]", "test")
-    assert(sc.parallelize(1 to 2, 2).count() == 2)
-    resetSparkContext()
+    import SparkMasterRegex._
+
+    val masterStrings = Seq(
+      "local-cluster[2,1,1024]",
+      "local-cluster[2 , 1 , 1024]",
+      "local-cluster[2, 1, 1024]",
+      "local-cluster[ 2, 1, 1024 ]"
+    )
+
+    masterStrings.foreach {
+      case LOCAL_CLUSTER_REGEX(numSlaves, coresPerSlave, memoryPerSlave) =>
+        assert(numSlaves.toInt == 2)
+        assert(coresPerSlave.toInt == 1)
+        assert(memoryPerSlave.toInt == 1024)
+    }
   }
 
   test("simple groupByKey") {