In mediawiki-1.35.0-rc.3\tests\phpunit\includes\ExportTest.php, data is not parsed with enough sanitation.
The problem occurs on line 41 of https://phabricator.wikimedia.org/source/mediawiki/browse/master/tests/phpunit/includes/ExportTest.php$34 :
34 $sink = new DumpStringOutput; 35 $exporter->setOutputSink( $sink ); 36 $exporter->openStream(); 37 $exporter->pageByTitle( $title ); 38 $exporter->closeStream(); 39 40 // This throws error if invalid xml output 41 $xmlObject = simplexml_load_string( $sink );