Fix phpunit issue from customfield_category with other plugin

[TomoTsuyuki]

PHPUnit error occurs when testing with other plugin.

1) mod_cms\datasource_userlist_test::test_import
Failed asserting that 2 matches expected 1.

/var/www/mdl41-monash/mod/cms/tests/datasource_userlist_test.php:131
/var/www/mdl41-monash/mod/cms/tests/datasource_userlist_test.php:106
/var/www/mdl41-monash/lib/phpunit/classes/advanced_testcase.php:80
phpvfscomposer:///var/www/mdl41-monash/vendor/phpunit/phpunit/phpunit:97

It looks there is another record from customfield_category.