Build/Test Tools: Use spl_object_id() in _test_filter_build_unique_id().
spl_object_hash() has been deprecated in PHP 8.6. tests_add_filter() uses this helper to build callback IDs before WordPress is loaded. A deprecation notice is triggered for plugin and theme test suites that register a closure or an object method during bootstrap.
The helper now builds the same IDs as _wp_filter_build_unique_id(), which switched to spl_object_id() in r62408 and began prefixing object IDs in r63334. The callbacks that run do not change because WP_Hook::build_preinitialized_hooks() re-adds every pre-loaded callback with add_filter().
A new test is included to confirm that both functions return the same ID for each type of callback.
Follow-up to r62408, r63334.
Developed in https://github.com/WordPress/wordpress-develop/pull/15625.
Props dhrupo.
Fixes #66512.
Built from https://develop.svn.wordpress.org/trunk@65314
![(please configure the [header_logo] section in trac.ini)](/chrome/site/your_project_logo.png){kind=logo}
