Only report not matching stderr content when we look for it

The previous test code could emit "D:cds:stderr did not match ''" rather
that just showing the contents of stderr.  Moved the debug line inside
the if/else block.

Replaced backquotes with $() and $(()) as approriate.

bin/tests/system/cds/tests.sh

@@ -17,7 +17,7 @@ status=0
 n=0
 fail() {
 	echo_i "failed"
-	status=`expr $status + 1`
+	status=$((status + 1))
 }
 
 runcmd() {
@@ -26,11 +26,11 @@ runcmd() {
 }
 
 testcase() {
-	n=`expr $n + 1`
+	n=$((n + 1))
 	echo_i "$name ($n)"
 	expect=$1
 	shift
-	result=`runcmd "$@"`
+	result=$(runcmd "$@")
 	check_stdout
 	check_stderr
 	if [ "$expect" -ne "$result" ]; then
@@ -43,10 +43,10 @@ testcase() {
 check_stderr() {
 	if [ -n "${err:=}" ]; then
 		egrep "$err" err.$n >/dev/null && return 0
+		echo_d "stderr did not match '$err'"
 	else
 		[ -s err.$n ] || return 0
 	fi
-	echo_d "stderr did not match '$err'"
 	cat err.$n | cat_d
 	fail
 }