ReferenceTester.php: Use extension's yaml_parse_file

In case of missing YAML library, the error message won't be the same.
Before, it pretended the use of `undefined function EzSystems\Raml2Html\Test\yaml_parse_file()`. After, it notifies the use of `undefined function yaml_parse_file()`.

Author: adriendupuis

tools/raml2html/src/Test/ReferenceTester.php

@@ -198,7 +198,7 @@ private function parseRoutingFiles($routingFiles): void
                 user_error("$routingFilePath doesn't exist or is not a file", E_USER_WARNING);
                 continue;
             }
-            $parsedRoutingFiles[$routingFile] = yaml_parse_file($routingFilePath);
+            $parsedRoutingFiles[$routingFile] = \yaml_parse_file($routingFilePath);
         }
 
         foreach ($parsedRoutingFiles as $routingFile => $parsedRoutingFile) {