allow testing lock code via async http calls

its hard to test code "stampede protection" code based on our locking framework without async http requests. two examples i've been working on:

• #848044: image_style_generate() should use lock_wait() →
• #802446: Cache stampede protection: drupal_render() and page cache →

in both cases, with async http calls, if we create a test module page that takes a long time to do the work in the lock (a big image, or a page/element that just has a sleep()), and fire multiple requests at it, we could test that the code works as expected without elaborate hacks.

as we require php 5.2 or greater, we have access to the curl_multi_* functions, so we can build this support into simpletest.