Don't call mb_strtolower() when generating machine names in Taxonomy tests

Reroll patch-4

Going by the problem statement upper case letters are allowed in machine names but there is a constraint which exists that doesn't allow this

core/modules/field/src/Entity/FieldStorageConfig.php

 if (!preg_match('/^[_a-z]+[_a-z0-9]*$/', $values['field_name'])) {
      throw new FieldException("Attempt to create a field storage {$values['field_name']} with invalid characters. Only lowercase alphanumeric characters and underscores are allowed, and only lowercase letters and underscore are allowed as the first character");
    }