[SPARK-32647][INFRA] Report SparkR test results with JUnit reporter

### What changes were proposed in this pull request?

This PR proposes to generate JUnit XML test report in SparkR tests that can be leveraged in both Jenkins and GitHub Actions.

**GitHub Actions**

![Screen Shot 2020-08-18 at 12 42 46 PM](https://user-images.githubusercontent.com/6477701/90467934-55b85b00-e150-11ea-863c-c8415e764ddb.png)

**Jenkins**

![Screen Shot 2020-08-18 at 2 03 42 PM](https://user-images.githubusercontent.com/6477701/90472509-a5505400-e15b-11ea-9165-777ec9b96eaa.png)

NOTE that while I am here, I am switching back the console reporter from "progress" to "summary". Currently non-ascii codes are broken in Jenkins console and switching it to "summary" can work around it.
"summary" is the default format used in testthat 1.x.

### Why are the changes needed?

To check the test failures more easily.

### Does this PR introduce _any_ user-facing change?

No, dev-only

### How was this patch tested?

It is tested in GitHub Actions at https://github.com/HyukjinKwon/spark/pull/23/checks?check_run_id=996586446
In case of Jenkins, https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/127525/testReport/

Closes #29456 from HyukjinKwon/sparkr-junit.

Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

R/pkg/tests/run-all.R

@@ -61,15 +61,18 @@ if (identical(Sys.getenv("NOT_CRAN"), "true")) {
     set.seed(42)
 
     # TODO (SPARK-30663) To be removed once testthat 1.x is removed from all builds
-    if (grepl("^1\\..*", packageVersion("testthat"))) {
+    if (packageVersion("testthat")$major <= 1) {
       # testthat 1.x
       test_runner <- testthat:::run_tests
       reporter <- "summary"
-
     } else {
       # testthat >= 2.0.0
       test_runner <- testthat:::test_package_dir
-      reporter <- testthat::default_reporter()
+      dir.create("target/test-reports", showWarnings = FALSE)
+      reporter <- MultiReporter$new(list(
+        SummaryReporter$new(),
+        JunitReporter$new(file = "target/test-reports/test-results.xml")
+      ))
     }
 
     test_runner("SparkR",

appveyor.yml

@@ -41,8 +41,8 @@ cache:
 install:
   # Install maven and dependencies
   - ps: .\dev\appveyor-install-dependencies.ps1
-  # Required package for R unit tests
-  - cmd: Rscript -e "install.packages(c('knitr', 'rmarkdown', 'testthat', 'e1071', 'survival', 'arrow'), repos='https://cloud.r-project.org/')"
+  # Required package for R unit tests. xml2 is required to use jUnit reporter in testthat.
+  - cmd: Rscript -e "install.packages(c('knitr', 'rmarkdown', 'testthat', 'e1071', 'survival', 'arrow', 'xml2'), repos='https://cloud.r-project.org/')"
   - cmd: Rscript -e "pkg_list <- as.data.frame(installed.packages()[,c(1, 3:4)]); pkg_list[is.na(pkg_list$Priority), 1:2, drop = FALSE]"
 
 build_script: