Use typed properties in OmniTools tests

Summary:
PHP 7.4 introduced typed properties. As we run tests under PHPUnit 10,
tests suite requires PHP 8.1+. As such, we can use typed properties.

Test Plan: phpunit omnitools/tests/

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3582