[MINOR][DOCS] Improve Running R Tests docs

## What changes were proposed in this pull request?

Update Running R Tests dependence packages to:
```bash
R -e "install.packages(c('knitr', 'rmarkdown', 'testthat', 'e1071', 'survival'), repos='http://cran.us.r-project.org')"
```

## How was this patch tested?
manual tests

Author: Yuming Wang <wgyumg@gmail.com>

Closes #18271 from wangyum/building-spark.

R/README.md

@@ -66,11 +66,7 @@ To run one of them, use `./bin/spark-submit <filename> <args>`. For example:
 ```bash
 ./bin/spark-submit examples/src/main/r/dataframe.R
 ```
-You can also run the unit tests for SparkR by running. You need to install the [testthat](http://cran.r-project.org/web/packages/testthat/index.html) package first:
-```bash
-R -e 'install.packages("testthat", repos="http://cran.us.r-project.org")'
-./R/run-tests.sh
-```
+You can run R unit tests by following the instructions under [Running R Tests](http://spark.apache.org/docs/latest/building-spark.html#running-r-tests).
 
 ### Running on YARN
 

R/WINDOWS.md

@@ -34,10 +34,9 @@ To run the SparkR unit tests on Windows, the following steps are required —ass
 
 4. Set the environment variable `HADOOP_HOME` to the full path to the newly created `hadoop` directory.
 
-5. Run unit tests for SparkR by running the command below. You need to install the [testthat](http://cran.r-project.org/web/packages/testthat/index.html) package first:
+5. Run unit tests for SparkR by running the command below. You need to install the needed packages following the instructions under [Running R Tests](http://spark.apache.org/docs/latest/building-spark.html#running-r-tests) first:
 
     ```
-    R -e "install.packages('testthat', repos='http://cran.us.r-project.org')"
     .\bin\spark-submit2.cmd --conf spark.hadoop.fs.defaultFS="file:///" R\pkg\tests\run-all.R
     ```
 

docs/building-spark.md

@@ -218,9 +218,11 @@ The run-tests script also can be limited to a specific Python version or a speci
 
 ## Running R Tests
 
-To run the SparkR tests you will need to install the R package `testthat`
-(run `install.packages(testthat)` from R shell).  You can run just the SparkR tests using
-the command:
+To run the SparkR tests you will need to install the [knitr](https://cran.r-project.org/package=knitr), [rmarkdown](https://cran.r-project.org/package=rmarkdown), [testthat](https://cran.r-project.org/package=testthat), [e1071](https://cran.r-project.org/package=e1071) and [survival](https://cran.r-project.org/package=survival) packages first:
+
+    R -e "install.packages(c('knitr', 'rmarkdown', 'testthat', 'e1071', 'survival'), repos='http://cran.us.r-project.org')"
+
+You can run just the SparkR tests using the command:
 
     ./R/run-tests.sh