calling self::getDataGenerator()->create_user() can generate username as 翔佐藤

This is not a standard username. It will fail validation against PARAM_USERNAME.

The bug error is due to the concatenation of firstname /lastname:

$record['username'] = textlib::strtolower($record['firstname']).textlib::strtolower($record['lastname']);

A quick fix is to clean_param and to incremente to $username.'XX'. It should be fine as these fake users should be cleaned at the end of the PHPunit tests.