itest: fix log whitelist check

Because a previous PR changed the location of the itest log files, we
also need to update the script that looks for non-whitelisted errors.

lntest/itest/log_check_errors.sh

@@ -5,7 +5,7 @@ BASEDIR=$(dirname "$0")
 echo ""
 
 # Filter all log files for errors, substitute variable data and match against whitelist.
-cat $BASEDIR/*.log | grep "\[ERR\]" | \
+find $BASEDIR -name "*.log" | xargs grep -h "\[ERR\]" | \
 sed -r -f $BASEDIR/log_substitutions.txt | \
 sort | uniq | \
 grep -Fvi -f $BASEDIR/log_error_whitelist.txt