test: refactor test-net-dns-custom-lookup

Use asssert.strictEqual to disallow coersion.

PR-URL: https://github.com/nodejs/node/pull/10071
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-net-dns-custom-lookup.js

@@ -18,9 +18,9 @@ function check(addressType, cb) {
       family: addressType,
       lookup: lookup
     }).on('lookup', common.mustCall(function(err, ip, type) {
-      assert.equal(err, null);
+      assert.strictEqual(err, null);
-      assert.equal(address, ip);
+      assert.strictEqual(address, ip);
-      assert.equal(type, addressType);
+      assert.strictEqual(type, addressType);
     }));
   }));