[SPARK-16005][R] Add `randomSplit` to SparkR

## What changes were proposed in this pull request?

This PR adds `randomSplit` to SparkR for API parity.

## How was this patch tested?

Pass the Jenkins tests (with new testcase.)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13721 from dongjoon-hyun/SPARK-16005.