[SPARK-6881][SparkR] Changes the checkpoint directory name.

Author: hlin09 <hlin09pu@gmail.com>

Closes #5493 from hlin09/fixCheckpointDir and squashes the following commits:

e67fc40 [hlin09] Change to temp dir.
1f7ed9e [hlin09] Change the checkpoint dir name.

R/pkg/R/RDD.R

@@ -279,7 +279,7 @@ setMethod("unpersist",
 #' @examples
 #'\dontrun{
 #' sc <- sparkR.init()
-#' setCheckpointDir(sc, "checkpoints")
+#' setCheckpointDir(sc, "checkpoint")
 #' rdd <- parallelize(sc, 1:10, 2L)
 #' checkpoint(rdd)
 #'}

R/pkg/R/context.R

@@ -216,7 +216,7 @@ broadcast <- function(sc, object) {
 #' @examples
 #'\dontrun{
 #' sc <- sparkR.init()
-#' setCheckpointDir(sc, "~/checkpoints")
+#' setCheckpointDir(sc, "~/checkpoint")
 #' rdd <- parallelize(sc, 1:2, 2L)
 #' checkpoint(rdd)
 #'}

R/pkg/inst/tests/test_rdd.R

@@ -141,7 +141,8 @@ test_that("PipelinedRDD support actions: cache(), persist(), unpersist(), checkp
   unpersist(rdd2)
   expect_false(rdd2@env$isCached)
 
-  setCheckpointDir(sc, "checkpoints")
+  tempDir <- tempfile(pattern = "checkpoint")
+  setCheckpointDir(sc, tempDir)
   checkpoint(rdd2)
   expect_true(rdd2@env$isCheckpointed)
 
@@ -152,7 +153,7 @@ test_that("PipelinedRDD support actions: cache(), persist(), unpersist(), checkp
   # make sure the data is collectable
   collect(rdd2)
 
-  unlink("checkpoints")
+  unlink(tempDir)
 })
 
 test_that("reduce on RDD", {