[SPARK-20626][SPARKR] address date test warning with timezone on windows

## What changes were proposed in this pull request?

set timezone on windows

## How was this patch tested?

unit test, AppVeyor

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #17892 from felixcheung/rtimestamptest.

R/pkg/inst/tests/testthat/test_sparkSQL.R

@@ -96,6 +96,10 @@ mockLinesMapType <- c("{\"name\":\"Bob\",\"info\":{\"age\":16,\"height\":176.5}}
 mapTypeJsonPath <- tempfile(pattern = "sparkr-test", fileext = ".tmp")
 writeLines(mockLinesMapType, mapTypeJsonPath)
 
+if (.Platform$OS.type == "windows") {
+  Sys.setenv(TZ = "GMT")
+}
+
 test_that("calling sparkRSQL.init returns existing SQL context", {
   skip_on_cran()