Merge pull request #469 from ajtulloch/use-local-spark-context-in-tests-for-mllib

[MLlib] Use a LocalSparkContext trait in test suites

Replaces the 9 instances of

```scala
class XXXSuite extends FunSuite with BeforeAndAfterAll {
  @transient private var sc: SparkContext = _

  override def beforeAll() {
    sc = new SparkContext("local", "test")
  }

  override def afterAll() {
    sc.stop()
    System.clearProperty("spark.driver.port")
  }
```

with

```scala
class XXXSuite extends FunSuite with LocalSparkContext {
```