[MINOR][MLLIB] fix mllib compile warnings

This PR fixes some warnings found by `build/sbt mllib/test:compile`.

Author: Xiangrui Meng <meng@databricks.com>

Closes #11227 from mengxr/fix-mllib-warnings-201602.

mllib/src/test/scala/org/apache/spark/ml/classification/DecisionTreeClassifierSuite.scala

@@ -305,7 +305,11 @@ class DecisionTreeClassifierSuite extends SparkFunSuite with MLlibTestSparkConte
         n.split match {
           case s: CategoricalSplit =>
             assert(s.leftCategories === Array(1.0))
+          case other =>
+            fail(s"All splits should be categorical, but got ${other.getClass.getName}: $other.")
         }
+      case other =>
+        fail(s"Root node should be an internal node, but got ${other.getClass.getName}: $other.")
     }
   }
 

mllib/src/test/scala/org/apache/spark/mllib/fpm/FPGrowthSuite.scala

@@ -16,6 +16,8 @@
  */
 package org.apache.spark.mllib.fpm
 
+import scala.language.existentials
+
 import org.apache.spark.SparkFunSuite
 import org.apache.spark.mllib.util.MLlibTestSparkContext
 import org.apache.spark.util.Utils