[SPARK-2417][MLlib] Fix DecisionTree tests

Fixes test failures introduced by apache#1316.

For both the regression and classification cases,
val stats is the InformationGainStats for the best tree split.
stats.predict is the predicted value for the data, before the split is made.
Since 600 of the 1,000 values generated by DecisionTreeSuite.generateCategoricalDataPoints() are 1.0 and the rest 0.0, the regression tree and classification tree both correctly predict a value of 0.6 for this data now, and the assertions have been changed to reflect that.

Author: johnnywalleye <[email protected]>

Closes apache#1343 from johnnywalleye/decision-tree-tests and squashes the following commits:

ef80603 [johnnywalleye] [SPARK-2417][MLlib] Fix DecisionTree tests

mllib/src/test/scala/org/apache/spark/mllib/tree/DecisionTreeSuite.scala

@@ -253,8 +253,8 @@ class DecisionTreeSuite extends FunSuite with LocalSparkContext {
 
     val stats = bestSplits(0)._2
     assert(stats.gain > 0)
-    assert(stats.predict > 0.4)
-    assert(stats.predict < 0.5)
+    assert(stats.predict > 0.5)
+    assert(stats.predict < 0.7)
     assert(stats.impurity > 0.2)
   }
 
@@ -280,8 +280,8 @@ class DecisionTreeSuite extends FunSuite with LocalSparkContext {
 
     val stats = bestSplits(0)._2
     assert(stats.gain > 0)
-    assert(stats.predict > 0.4)
-    assert(stats.predict < 0.5)
+    assert(stats.predict > 0.5)
+    assert(stats.predict < 0.7)
     assert(stats.impurity > 0.2)
   }