JIRA: https://issues.apache.org/jira/browse/SPARK-13472

## What changes were proposed in this pull request?

One Kmeans test in R is unstable and sometimes fails. We should fix it.

## How was this patch tested?

Unit test is modified in this PR.

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

Closes #11345 from viirya/fix-kmeans-r-test and squashes the following commits:

f959f61 [Liang-Chi Hsieh] Sort resulted clusters.