test: remove flag for test-addon-uv-handle-leak

test-addon-uv-handle-leak only requires worker_threads for the
subprocess which it explicitly calls with --experimental-worker. The
main test itself does not need it. Remove Flags: comment and move
loading of worker_threads into subprocess-only logic.

PR-URL: #25327
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/abort/test-addon-uv-handle-leak.js

@@ -1,11 +1,9 @@
-// Flags: --experimental-worker
 'use strict';
 const common = require('../common');
 const assert = require('assert');
 const fs = require('fs');
 const path = require('path');
 const cp = require('child_process');
-const { Worker } = require('worker_threads');
 const { spawnSync } = require('child_process');
 
 // This is a sibling test to test/addons/uv-handle-leak.
@@ -19,6 +17,8 @@ if (!fs.existsSync(bindingPath))
 
 if (process.argv[2] === 'child') {
 
+  const { Worker } = require('worker_threads');
+
   // The worker thread loads and then unloads `bindingPath`. Because of this the
   // symbols in `bindingPath` are lost when the worker thread quits, but the
   // number of open handles in the worker thread's event loop is assessed in the