test: add test for dns.promises.resolve .

PR-URL: https://github.com/nodejs/node/pull/21691
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-dns-promises-resolve.js

@@ -0,0 +1,44 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+
+const dnsPromises = require('dns').promises;
+
+common.crashOnUnhandledRejection();
+
+// Error when rrtype is invalid.
+{
+  const rrtype = 'DUMMY';
+  common.expectsError(
+    () => dnsPromises.resolve('example.org', rrtype),
+    {
+      code: 'ERR_INVALID_OPT_VALUE',
+      type: TypeError,
+      message: `The value "${rrtype}" is invalid for option "rrtype"`
+    }
+  );
+}
+
+// Error when rrtype is a number.
+{
+  const rrtype = 0;
+  common.expectsError(
+    () => dnsPromises.resolve('example.org', rrtype),
+    {
+      code: 'ERR_INVALID_ARG_TYPE',
+      type: TypeError,
+      message: 'The "rrtype" argument must be of type string. ' +
+               `Received type ${typeof rrtype}`
+    }
+  );
+}
+
+// rrtype is undefined, it's same as resolve4
+{
+  (async function() {
+    const rrtype = undefined;
+    const result = await dnsPromises.resolve('example.org', rrtype);
+    assert.ok(result !== undefined);
+    assert.ok(result.length > 0);
+  })();
+}