test: improve flaky test-listen-fd-ebadf.js

Find an invalid file descriptor rather than assuming 42 will be invalid.

Fixes: #17762

test/parallel/test-listen-fd-ebadf.js

@@ -21,12 +21,24 @@
 
 'use strict';
 const common = require('../common');
+
 const assert = require('assert');
+const fs = require('fs');
 const net = require('net');
 
 net.createServer(common.mustNotCall()).listen({ fd: 2 })
   .on('error', common.mustCall(onError));
-net.createServer(common.mustNotCall()).listen({ fd: 42 })
+
+let invalidFd = 2;
+
+// Get first known bad file descriptor.
+try {
+  while (fs.fstatSync(++invalidFd));
+} catch (e) {
+  // do nothing; we now have an invalid fd
+}
+
+net.createServer(common.mustNotCall()).listen({ fd: invalidFd })
   .on('error', common.mustCall(onError));
 
 function onError(ex) {