Use arithmetic expansion in system tests

Change the way arithmetic operations are performed in system test shell
scripts from using `expr` to `$(())`. This ensures that updating the
variable won't end up with a non-zero exit code, which would case the
script to exit prematurely when `set -e` is in effect.

The following replacements were performed using sed in all text files
(git grep -Il '' | xargs sed -i):

s/status=`expr $status + $ret`/status=$((status + ret))/g
s/n=`expr $n + 1`/n=$((n + 1))/g
s/t=`expr $t + 1`/t=$((t + 1))/g
s/status=`expr $status + 1`/status=$((status + 1))/g
s/try=`expr $try + 1`/try=$((try + 1))/g

bin/tests/system/names/tests.sh

@@ -35,14 +35,14 @@ echo_i "Checking if responses are identical other than in message size"
 diff dig.compdis.sorted.test dig.compen.sorted.test >/dev/null
 ret=$?
 if [ $ret != 0 ]; then echo_i "failed"; fi
-status=`expr $status + $ret`
+status=$((status + ret))
 
 echo_i "Checking if message with compression disabled is significantly larger"
 echo_i "Disabled $COMPDIS vs enabled $COMPEN"
 val=`expr \( $COMPDIS \* 3 / 2 \) / $COMPEN`
 if [ $val -le 1 ]; then
 	echo_i "failed"
-	status=`expr $status + 1`
+	status=$((status + 1))
 fi;
 
 echo_i "exit status: $status"