test: add IPv6 brackets but no port to test-dns

Add a test case to test-dns to check that supply an IPv6 host with
brackets but no explicit port to `dns.setServers()` yields expected
results. This is the final bit of test coverage missing for
lib/internal/dns/utils.js.

PR-URL: #27006
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Signed-off-by: Beth Griggs <Bethany.Griggs@uk.ibm.com>

Author: Trott

test/parallel/test-dns.js

@@ -121,13 +121,15 @@ const ports = [
   '4.4.4.4:53',
   '[2001:4860:4860::8888]:53',
   '103.238.225.181:666',
-  '[fe80::483a:5aff:fee6:1f04]:666'
+  '[fe80::483a:5aff:fee6:1f04]:666',
+  '[fe80::483a:5aff:fee6:1f04]',
 ];
 const portsExpected = [
   '4.4.4.4',
   '2001:4860:4860::8888',
   '103.238.225.181:666',
-  '[fe80::483a:5aff:fee6:1f04]:666'
+  '[fe80::483a:5aff:fee6:1f04]:666',
+  'fe80::483a:5aff:fee6:1f04',
 ];
 dns.setServers(ports);
 assert.deepStrictEqual(dns.getServers(), portsExpected);