run-test: pass --experimental-test-module-mocks through to node

Specs that replace a module need mock.module from node:test, which
Node only exposes behind this flag. The runner rejected unknown flags,
so there was no way to turn it on. It is now accepted and forwarded to
node like --import and --require.

The repo's own test run enables it, and a spec checks that a module
registered as a mock is what a later dynamic import receives.

packages/dev-test/src/env/moduleMocks.spec.ts

@@ -0,0 +1,14 @@
+// Copyright 2017-2026 @pezkuwi/dev-test authors & contributors
+// SPDX-License-Identifier: Apache-2.0
+
+import { mock } from 'node:test';
+
+describe('mock.module', () => {
+  it('replaces a module loaded after the mock is registered', async () => {
+    mock.module('node:os', { namedExports: { platform: () => 'mocked' } });
+
+    const { platform } = await import('node:os');
+
+    expect(platform()).toBe('mocked');
+  });
+});