Replace a seq invocation with a shell loop

seq is not portable.  Use a while loop instead to make the "dnssec"
system test script POSIX-compatible.

(cherry picked from commit c0c4c024c6)

bin/tests/system/dnssec/ns2/sign.sh

@@ -196,8 +196,10 @@ cat > "$zonefile" << EOF
 ns2	10	A	10.53.0.2
 ns3	10	A	10.53.0.3
 EOF
-for i in $(seq 300); do
+i=1
+while [ $i -le 300 ]; do
     echo "host$i 10 IN NS ns.elsewhere"
+    i=$((i+1))
 done >> "$zonefile"
 key1=$("$KEYGEN" -q -a "$DEFAULT_ALGORITHM" -b "$DEFAULT_BITS" -n zone -f KSK "$zone")
 key2=$("$KEYGEN" -q -a "$DEFAULT_ALGORITHM" -b "$DEFAULT_BITS" -n zone "$zone")