Resolve localhost to both loopbacks in the real connect-failure test

The FIRMS test that checks a real all-addresses connect failure relied on
the system resolving localhost to both ::1 and 127.0.0.1. On a host whose
localhost resolves only to 127.0.0.1, fetch makes a single attempt, gets a
plain ECONNREFUSED instead of an AggregateError, and the test fails.

Mock dns.lookup for that fetch to return both loopback addresses, so the
real fetch still attempts both families and the error shape check stays
deterministic. Assert the mock was used, so a runtime that stops resolving
through dns.lookup fails loudly instead of silently testing something else.

src/data/firmsProxy.test.mjs

@@ -3,6 +3,7 @@ import fs from 'node:fs';
 import test from 'node:test';
 import { EventEmitter } from 'node:events';
 import dns from 'node:dns/promises';
+import dnsLookup from 'node:dns';
 import https from 'node:https';
 import net from 'node:net';
 import { Readable } from 'node:stream';
@@ -258,7 +259,8 @@ test('the IPv4 retry answering an invalid reason phrase rejects instead of crash
 
 // The fixtures above copy Node's error shape; check it against a real fetch so
 // a Node or undici change that stops matching fails here instead of silently
-// disabling the retry. A closed port on `localhost` refuses both addresses.
+// disabling the retry. A closed port refuses both loopback addresses; resolve
+// `localhost` to both here, since some hosts map it to 127.0.0.1 only.
 test('a real all-addresses connect failure is recognised as address selection', async (t) => {
   const probe = net.createServer();
   try {
@@ -272,7 +274,10 @@ test('a real all-addresses connect failure is recognised as address selection',
   const v4 = net.createServer();
   await new Promise((resolve, reject) => v4.once('error', reject).listen(port, '127.0.0.1', resolve));
   await new Promise((resolve) => v4.close(resolve));
+  const loopback = [{ address: '::1', family: 6 }, { address: '127.0.0.1', family: 4 }];
+  const lookup = t.mock.method(dnsLookup, 'lookup', (_host, _options, callback) => callback(null, loopback));
   const err = await fetch(`http://localhost:${port}/`).then(() => null, (error) => error);
+  assert.equal(lookup.mock.callCount(), 1, 'fetch resolves localhost through dns.lookup');
   assert.ok(err, 'a closed port refuses the connection');
   assert.equal(isAddressSelectionFailure(err), true, describeFetchError(err));
   assert.match(describeFetchError(err), /ECONNREFUSED .*; .*ECONNREFUSED/);