Fix constants being redefined in PHP 8.4 polyfill

[cs278]

Tracked down this error to this library:

In bootstrap.php line 19:

  [ErrorException]
  Constant CURL_HTTP_VERSION_3 already defined

It appears to me the standard is to conditionally define symbols so these two were missing a check. I've also added a basic test which should prevent this happening again.

[nicolas-grekas]

Thank you @cs278.