[SPARK-49347][R][FOLLOW-UP] Set SPARK_TESTING in Windows build#47881

HyukjinKwon wants to merge 1 commit intoapache:masterapache/spark:masterfrom

HyukjinKwon:SPARK-49347-followupHyukjinKwon/spark:SPARK-49347-followupCopy head branch name to clipboard