Replace Fixture Listener with schema loader

[Staubiii]

TestListenerTrait was removed. PHPUnit dropped support for these listeners. See PHPUnit 10 Upgrade

https://book.cakephp.org/5/en/appendices/5-0-migration-guide.html#testsuite

This leads to a deprecation message with newer PHPUnit 9.x versions:

composer test
phpunit
Deprecated: 4.5.0 - TestListenerTrait is deprecated, as PHPUnit is removing support for listeners.

Necessary changes:
cakephp/debug_kit@8a2ad6a#diff-e35810879ec42bdd81797b3ccb72f6de28a8b4a0e3bfdba43183e133e866b892

[ravage84]

Would this plugin ever use fixtures? If not, we could just remove the problematic functionality.