test: fix flaky test-worker-ref-onexit

Fixes: #26167

PR-URL: #26170
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>

test/parallel/test-worker-ref-onexit.js

@@ -5,6 +5,8 @@ const { Worker } = require('worker_threads');
 // Check that worker.unref() makes the 'exit' event not be emitted, if it is
 // the only thing we would otherwise be waiting for.
 
-const w = new Worker('', { eval: true });
+// Use `setInterval()` to make sure the worker is alive until the end of the
+// event loop turn.
+const w = new Worker('setInterval(() => {}, 100);', { eval: true });
 w.unref();
 w.on('exit', common.mustNotCall());