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.

git-svn-id: https://develop.svn.wordpress.org/trunk@59108 602fd350-edb4-49c9-b593-d223f7449a82

Author: hellofromtonya

tests/phpunit/includes/class-wp-test-stream.php

@@ -57,7 +57,7 @@ private function open( $url ) {
 		$this->file   = $components['path'] ? $components['path'] : '/';
 
 		if ( empty( $this->bucket ) ) {
-			trigger_error( 'Cannot use an empty bucket name', E_USER_ERROR );
+			throw new Exception( 'Cannot use an empty bucket name' );
 		}
 
 		if ( ! isset( WP_Test_Stream::$data[ $this->bucket ] ) ) {