Ensure tests can run with PHPUnit 9
Summary:
Fixes the following issue:
PHP Fatal error: Declaration of Keruald\OmniTools\Tests\Strings\Multibyte\OmniStringTest::setUp() must be compatible with PHPUnit\Framework\TestCase::setUp(): void in .../keruald/omnitools/tests/Strings/Multibyte/OmniStringTest.php on line 97
Test Plan: phpunit
Reviewers: dereckson
Reviewed By: dereckson
Differential Revision: https://devcentral.nasqueron.org/D2274