## What changes were proposed in this pull request?
This is a followup to reduce AppVeyor test time. This PR proposes to reduce the number of shuffle partitions to reduce the tasks running R workers in few particular tests.
The symptom is similar as described in `https://github.com/apache/spark/pull/19722`. There are many R processes newly launched on Windows without forking and it makes the differences of elapsed time between Linux and Windows.
Here is the simple comparison for before/after of this change. I manually tested this by disabling `spark.sparkr.use.daemon`. Disabling it resembles the tests on Windows:
**Before**
<img width="672" alt="2017-11-25 12 22 13" src="https://user-images.githubusercontent.com/6477701/33217949-b5528dfa-d17d-11e7-8050-75675c39eb20.png">
**After**
<img width="682" alt="2017-11-25 12 32 00" src="https://user-images.githubusercontent.com/6477701/33217958-c6518052-d17d-11e7-9f8e-1be21a784559.png">
So, this probably will reduce roughly more than 10 minutes.
## How was this patch tested?
AppVeyor tests
Author: hyukjinkwon <gurwls223@gmail.com>
Closes#19816 from HyukjinKwon/SPARK-21693-followup.
d49d9e4038
