test: fix flaky test-fs-promises-file-handle-close

Sometimes, the expected warnings were not emitted.
To harden the test:

- Instead of relying on a timeout that raced against the file being
  opened, wait until the file is opened and then schedule GC
  after that.
- Explicitly keep the event loop alive for one turn to make sure
  the warning is being seen.

PR-URL: #31687
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-fs-promises-file-handle-close.js

@@ -27,13 +27,15 @@ async function doOpen() {
   return fh;
 }
 
-// Perform the file open assignment within a block.
-// When the block scope exits, the file handle will
-// be eligible for garbage collection.
-{
-  doOpen().then(common.mustCall((fd) => {
-    assert.strictEqual(typeof fd, 'object');
-  }));
-}
-
-setTimeout(() => global.gc(), 10);
+doOpen().then(common.mustCall((fd) => {
+  assert.strictEqual(typeof fd, 'object');
+})).then(common.mustCall(() => {
+  setImmediate(() => {
+    // The FileHandle should be out-of-scope and no longer accessed now.
+    global.gc();
+
+    // Wait an extra event loop turn, as the warning is emitted from the
+    // native layer in an unref()'ed setImmediate() callback.
+    setImmediate(common.mustCall());
+  });
+}));