[SPARK-20543][SPARKR] skip tests when running on CRAN

## What changes were proposed in this pull request?

General rule on skip or not:
skip if
- RDD tests
- tests could run long or complicated (streaming, hivecontext)
- tests on error conditions
- tests won't likely change/break

## How was this patch tested?

unit tests, `R CMD check --as-cran`, `R CMD check`

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #17817 from felixcheung/rskiptest.