Replace deprecated setMethods with onlyMethods

[djarran]

Description: Replace deprecated setMethods with onlyMethods. Test still runs successfully in Moodle 4.5:

root@2148dbd199be:/var/www/cat-moo# vendor/bin/phpunit admin/tool/emailutils/tests/suffix_test.php
Moodle 4.5.11+ (Build: 20260501), 38becfa44d6eaaf99250c36f3848b9d76e135a78
Php: 8.3.31, pgsql: 17.10 (Debian 17.10-1.pgdg13+1), OS: Linux 6.17.0-29-generic x86_64
PHPUnit 9.6.34 by Sebastian Bergmann and contributors.

.............                                                     13 / 13 (100%)

Time: 00:01.422, Memory: 48.00 MB

OK (13 tests, 13 assertions)