[SPARK-25273][DOC] How to install testthat 1.0.2

## What changes were proposed in this pull request?

R tests require `testthat` v1.0.2. In the PR, I described how to install the version in the section http://spark.apache.org/docs/latest/building-spark.html#running-r-tests.

Closes apache#22272 from MaxGekk/r-testthat-doc.

Authored-by: Maxim Gekk <[email protected]>
Signed-off-by: hyukjinkwon <[email protected]>

Author: MaxGekk

docs/README.md

@@ -22,8 +22,9 @@ $ sudo gem install jekyll jekyll-redirect-from pygments.rb
 $ sudo pip install Pygments
 # Following is needed only for generating API docs
 $ sudo pip install sphinx pypandoc mkdocs
-$ sudo Rscript -e 'install.packages(c("knitr", "devtools", "testthat", "rmarkdown"), repos="http://cran.stat.ucla.edu/")'
+$ sudo Rscript -e 'install.packages(c("knitr", "devtools", "rmarkdown"), repos="http://cran.stat.ucla.edu/")'
 $ sudo Rscript -e 'devtools::install_version("roxygen2", version = "5.0.1", repos="http://cran.stat.ucla.edu/")'
+$ sudo Rscript -e 'devtools::install_version("testthat", version = "1.0.2", repos="http://cran.stat.ucla.edu/")'
 ```
 
 Note: If you are on a system with both Ruby 1.9 and Ruby 2.0 you may need to replace gem with gem2.0.

docs/building-spark.md

@@ -236,7 +236,8 @@ The run-tests script also can be limited to a specific Python version or a speci
 
 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')"
+    R -e "install.packages(c('knitr', 'rmarkdown', 'devtools', 'e1071', 'survival'), repos='http://cran.us.r-project.org')"
+    R -e "devtools::install_version('testthat', version = '1.0.2', repos='http://cran.us.r-project.org')"
 
 You can run just the SparkR tests using the command: