[WIP] Create tests for custom checkout permalink

[ajello-meta]

Changes proposed in this Pull Request:

This PR introduces unit tests for the custom checkout permalink in the WooCommerce\Facebook\Checkout namespace. These tests ensure that the class behaves as expected when handling hooks, rewrite rules, and query variables. It also ensures that the correct checkout experience is loaded.

Detailed test instructions:

1. Run the unit tests using the WordPress testing framework to ensure they pass successfully.
2. Verify that the hooks were added correctly.
3. Verify that the rewrite rules were registered correctly.
4. Verify that the query parameters were parsed correctly.
5. Verify that the correct checkout experience is loaded.