[SPARK-17674][SPARKR] check for warning in test output

## What changes were proposed in this pull request?

testthat library we are using for testing R is redirecting warning (and disabling `options("warn" = 2)`), we need to have a way to detect any new warning and fail

## How was this patch tested?

manual testing, Jenkins

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #15576 from felixcheung/rtestwarning.

R/run-tests.sh

@@ -26,6 +26,8 @@ rm -f $LOGFILE
 SPARK_TESTING=1 $FWDIR/../bin/spark-submit --driver-java-options "-Dlog4j.configuration=file:$FWDIR/log4j.properties" --conf spark.hadoop.fs.default.name="file:///" $FWDIR/pkg/tests/run-all.R 2>&1 | tee -a $LOGFILE
 FAILED=$((PIPESTATUS[0]||$FAILED))
 
+NUM_TEST_WARNING="$(grep -c -e 'Warnings ----------------' $LOGFILE)"
+
 # Also run the documentation tests for CRAN
 CRAN_CHECK_LOG_FILE=$FWDIR/cran-check.out
 rm -f $CRAN_CHECK_LOG_FILE
@@ -37,10 +39,10 @@ NUM_CRAN_WARNING="$(grep -c WARNING$ $CRAN_CHECK_LOG_FILE)"
 NUM_CRAN_ERROR="$(grep -c ERROR$ $CRAN_CHECK_LOG_FILE)"
 NUM_CRAN_NOTES="$(grep -c NOTE$ $CRAN_CHECK_LOG_FILE)"
 
-if [[ $FAILED != 0 ]]; then
+if [[ $FAILED != 0 || $NUM_TEST_WARNING != 0 ]]; then
     cat $LOGFILE
     echo -en "\033[31m"  # Red
-    echo "Had test failures; see logs."
+    echo "Had test warnings or failures; see logs."
     echo -en "\033[0m"  # No color
     exit -1
 else