[SPARK-20704][SPARKR] change CRAN test to run single thread

## What changes were proposed in this pull request?

- [x] need to test by running R CMD check --as-cran
- [x] sanity check vignettes

## How was this patch tested?

Jenkins

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #17945 from felixcheung/rchangesforpackage.