[FIX] disable benchmark code

I forgot to disable the benchmark code in #2937, which increased the Jenkins build time by couple minutes.

aarondav

Author: Xiangrui Meng <meng@databricks.com>

Closes #2990 from mengxr/disable-benchmark and squashes the following commits:

c58f070 [Xiangrui Meng] disable benchmark code

core/src/test/scala/org/apache/spark/util/collection/SorterSuite.scala

@@ -157,7 +157,7 @@ class SorterSuite extends FunSuite {
    * here is mainly to have the code. Running multiple tests within the same JVM session would
    * prevent JIT inlining overridden methods and hence hurt the performance.
    */
-  test("Sorter benchmark for primitive int array") {
+  ignore("Sorter benchmark for primitive int array") {
     val numElements = 25000000 // 25 mil
     val rand = new XORShiftRandom(123)