[SPARK-19324][SPARKR] Spark VJM stdout output is getting dropped in SparkR

## What changes were proposed in this pull request?

This affects mostly running job from the driver in client mode when results are expected to be through stdout (which should be somewhat rare, but possible)

Before:
```
> a <- as.DataFrame(cars)
> b <- group_by(a, "dist")
> c <- count(b)
> sparkR.callJMethod(c$countjc, "explain", TRUE)
NULL
```

After:
```
> a <- as.DataFrame(cars)
> b <- group_by(a, "dist")
> c <- count(b)
> sparkR.callJMethod(c$countjc, "explain", TRUE)
count#11L
NULL
```

Now, `column.explain()` doesn't seem very useful (we can get more extensive output with `DataFrame.explain()`) but there are other more complex examples with calls of `println` in Scala/JVM side, that are getting dropped.

## How was this patch tested?

manual

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #16670 from felixcheung/rjvmstdout.

R/pkg/R/utils.R

@@ -756,12 +756,17 @@ varargsToJProperties <- function(...) {
   props
 }
 
-launchScript <- function(script, combinedArgs, capture = FALSE) {
+launchScript <- function(script, combinedArgs, wait = FALSE) {
   if (.Platform$OS.type == "windows") {
     scriptWithArgs <- paste(script, combinedArgs, sep = " ")
-    shell(scriptWithArgs, translate = TRUE, wait = capture, intern = capture) # nolint
+    # on Windows, intern = F seems to mean output to the console. (documentation on this is missing)
+    shell(scriptWithArgs, translate = TRUE, wait = wait, intern = wait) # nolint
   } else {
-    system2(script, combinedArgs, wait = capture, stdout = capture)
+    # http://stat.ethz.ch/R-manual/R-devel/library/base/html/system2.html
+    # stdout = F means discard output
+    # stdout = "" means to its console (default)
+    # Note that the console of this child process might not be the same as the running R process.
+    system2(script, combinedArgs, stdout = "", wait = wait)
   }
 }
 

R/pkg/inst/tests/testthat/test_Windows.R

@@ -20,7 +20,7 @@ test_that("sparkJars tag in SparkContext", {
   if (.Platform$OS.type != "windows") {
     skip("This test is only for Windows, skipped")
   }
-  testOutput <- launchScript("ECHO", "a/b/c", capture = TRUE)
+  testOutput <- launchScript("ECHO", "a/b/c", wait = TRUE)
   abcPath <- testOutput[1]
   expect_equal(abcPath, "a\\b\\c")
 })