test: fix flaky test-pipe-unref

PR-URL: #17950
Fixes: #16875
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-pipe-unref.js

@@ -2,10 +2,10 @@
 const common = require('../common');
 const net = require('net');
 
+// This test should end immediately after `unref` is called
+
 common.refreshTmpDir();
 
 const s = net.Server();
 s.listen(common.PIPE);
 s.unref();
-
-setTimeout(common.mustNotCall(), 1000).unref();