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/tools/tests.sh

@@ -19,7 +19,7 @@ checkout() {
 	case $? in
 	0) : ok ;;
 	*) echo_i "failed"
-	   status=`expr $status + 1`
+	   status=$((status + 1))
 	   return 1 ;;
 	esac
 	case $out in
@@ -27,7 +27,7 @@ checkout() {
 	*) echo_i "expect $hash"
 	   echo_i "output $out"
 	   echo_i "failed"
-	   status=`expr $status + 1` ;;
+	   status=$((status + 1)) ;;
 	esac
 }
 
@@ -86,7 +86,7 @@ checkempty
 checkfail() {
 	case $? in
 	0) echo_i "failed to fail"
-	   status=`expr $status + 1`
+	   status=$((status + 1))
 	   return 1 ;;
 	esac
 }