Add a test for NextIterator.

core/src/test/scala/spark/util/NextIteratorSuite.scala

+package spark.util
+
+import org.scalatest.FunSuite
+import org.scalatest.matchers.ShouldMatchers
+import scala.collection.mutable.Buffer
+import java.util.NoSuchElementException
+
+class NextIteratorSuite extends FunSuite with ShouldMatchers {
+  test("one iteration") {
+    val i = new StubIterator(Buffer(1))
+    i.hasNext should be === true
+    i.next should be === 1
+    i.hasNext should be === false
+    intercept[NoSuchElementException] { i.next() }
+  }
+  
+  test("two iterations") {
+    val i = new StubIterator(Buffer(1, 2))
+    i.hasNext should be === true
+    i.next should be === 1
+    i.hasNext should be === true
+    i.next should be === 2
+    i.hasNext should be === false
+    intercept[NoSuchElementException] { i.next() }
+  }
+
+  test("empty iteration") {
+    val i = new StubIterator(Buffer())
+    i.hasNext should be === false
+    intercept[NoSuchElementException] { i.next() }
+  }
+
+  test("close is called once for empty iterations") {
+    val i = new StubIterator(Buffer())
+    i.hasNext should be === false
+    i.hasNext should be === false
+    i.closeCalled should be === 1
+  }
+
+  test("close is called once for non-empty iterations") {
+    val i = new StubIterator(Buffer(1, 2))
+    i.next should be === 1
+    i.next should be === 2
+    // close isn't called until we check for the next element
+    i.closeCalled should be === 0
+    i.hasNext should be === false
+    i.closeCalled should be === 1
+    i.hasNext should be === false
+    i.closeCalled should be === 1
+  }
+
+  class StubIterator(ints: Buffer[Int])  extends NextIterator[Int] {
+    var closeCalled = 0
+    
+    override def getNext() = {
+      if (ints.size == 0) {
+        finished = true
+        0
+      } else {
+        ints.remove(0)
+      }
+    }
+
+    override def close() {
+      closeCalled += 1
+    }
+  }
+}