Add and use keyfile_to_key_id() helper function

When trying to extract the key ID from a key file name, some test code
incorrectly attempts to strip all leading zeros.  This breaks tests when
keys with ID 0 are generated.  Add a new helper shell function,
keyfile_to_key_id(), which properly handles keys with ID 0 and use it in
test code whenever a key ID needs to be extracted from a key file name.

bin/tests/system/mkeys/tests.sh

@@ -492,7 +492,7 @@ n=`expr $n + 1`
 echo_i "revoke key with bad signature, check revocation is ignored ($n)"
 ret=0
 revoked=`$REVOKE -K ns1 $original`
-rkeyid=`expr $revoked : 'ns1/K\.+00.+0*\([1-9]*[0-9]*[0-9]\)'`
+rkeyid=$(keyfile_to_key_id $revoked)
 rm -f ns1/root.db.signed.jnl
 # We need to activate at least one valid DNSKEY to prevent dnssec-signzone from
 # failing.  Alternatively, we could use -P to disable post-sign verification,