Fixed deprecation notices in tests

[blobaugh]

Description

The tests were throwing deprecation notices due to dynamic class properties being set. This PR resolves them by explicitly setting the properties.

Example of a notice that was thrown:

Deprecated: Creation of dynamic property WorkOS\WebhookTest::$payload is deprecated

Documentation

Does this require changes to the WorkOS Docs? E.g. the API Reference or code snippets need updates.

[ ] Yes

If yes, link a related docs PR and add a docs maintainer as a reviewer. Their approval is required.