[Minor][MLlib] Incorrect path to test data is used in DecisionTreeExample

It should load from `testInput` instead of `input` for test data.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #5594 from viirya/use_testinput and squashes the following commits:

5e8b174 [Liang-Chi Hsieh] Fix style.
b60b475 [Liang-Chi Hsieh] Use testInput.

Author: viirya

examples/src/main/scala/org/apache/spark/examples/ml/DecisionTreeExample.scala

@@ -173,7 +173,8 @@ object DecisionTreeExample {
     val splits: Array[RDD[LabeledPoint]] = if (testInput != "") {
       // Load testInput.
       val numFeatures = origExamples.take(1)(0).features.size
-      val origTestExamples: RDD[LabeledPoint] = loadData(sc, input, dataFormat, Some(numFeatures))
+      val origTestExamples: RDD[LabeledPoint] =
+        loadData(sc, testInput, dataFormat, Some(numFeatures))
       Array(origExamples, origTestExamples)
     } else {
       // Split input into training, test.