Code Modernization: Fix trigger_error() with E_USER_ERROR deprecation in TestXMLParser::parse().

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 [25002].

Props jrf.
See #62061.

git-svn-id: https://develop.svn.wordpress.org/trunk@59109 602fd350-edb4-49c9-b593-d223f7449a82

Author: hellofromtonya

tests/phpunit/includes/utils.php

@@ -304,13 +304,12 @@ public function __construct( $in ) {
 	public function parse( $in ) {
 		$parse = xml_parse( $this->xml, $in, true );
 		if ( ! $parse ) {
-			trigger_error(
+			throw new Exception(
 				sprintf(
 					'XML error: %s at line %d',
 					xml_error_string( xml_get_error_code( $this->xml ) ),
 					xml_get_current_line_number( $this->xml )
-				),
-				E_USER_ERROR
+				)
 			);
 			xml_parser_free( $this->xml );
 		}