[SPARK-20092][R][PROJECT INFRA] Add the detection for Scala codes dedicated for R in AppVeyor tests

## What changes were proposed in this pull request?

We are currently detecting the changes in `R/` directory only and then trigger AppVeyor tests.

It seems we need to tests when there are Scala codes dedicated for R in `core/src/main/scala/org/apache/spark/api/r/`, `sql/core/src/main/scala/org/apache/spark/sql/api/r/` and `mllib/src/main/scala/org/apache/spark/ml/r/` too.

This will enables the tests, for example, for SPARK-20088.

## How was this patch tested?

Tests with manually created PRs.

- Changes in `sql/core/src/main/scala/org/apache/spark/sql/api/r/SQLUtils.scala` https://github.com/spark-test/spark/pull/13
- Changes in `core/src/main/scala/org/apache/spark/api/r/SerDe.scala` https://github.com/spark-test/spark/pull/12
- Changes in `README.md` https://github.com/spark-test/spark/pull/14

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17427 from HyukjinKwon/SPARK-20092.