Code Modernization: Fix trigger_error() with E_USER_ERROR deprecation in WP_Test_Stream::open().

PHP 8.4 deprecates the use of `trigger_errror()` with `E_USER_ERROR` as the error level, as there are a number of gotchas to this way of creating a `Fatal Error` (`finally` blocks not executing, destructors not executing).
The recommended replacements are either to use exceptions or to do a hard `exit`.

As this is a test-only class, do not have to take BC-breaks into account.

Also, as this is a test helper, throwing a exception is the most appropriate solution.

Reference:
* https://wiki.php.net/rfc/deprecations_php_8_4#deprecate_passing_e_user_error_to_trigger_error

Follow-up to [49230].

Props jrf.
See #62061.
Built from https://develop.svn.wordpress.org/trunk@59108


git-svn-id: http://core.svn.wordpress.org/trunk@58504 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.7-alpha-59107';
+$wp_version = '6.7-alpha-59108';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.