[SPARK-21801][SPARKR][TEST] set random seed for predictable test

## What changes were proposed in this pull request?

set.seed() before running tests

## How was this patch tested?

jenkins, appveyor

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #19111 from felixcheung/rranseed.

R/pkg/tests/run-all.R

@@ -43,6 +43,8 @@ if (identical(Sys.getenv("NOT_CRAN"), "true")) {
 test_package("SparkR")
 
 if (identical(Sys.getenv("NOT_CRAN"), "true")) {
+  # set random seed for predictable results. mostly for base's sample() in tree and classification
+  set.seed(42)
   # for testthat 1.0.2 later, change reporter from "summary" to default_reporter()
   testthat:::run_tests("SparkR",
                        file.path(sparkRDir, "pkg", "tests", "fulltests"),