This commit adds four test cases (via a PHPUnit data provider) for the `wc_sanitize_order_id()` function.
While writing the tests, it was also discovered that the function was returning a *string*, not an integer as indicated by the DocBlock, so explicit type-casting was added.
[According to the WooCommerce wiki](https://github.com/woocommerce/woocommerce/wiki/Order-and-Order-Line-Item-Data) and `WC_REST_Orders_Controller`, WooCommerce stores country codes using the ISO 3166-1 alpha-2 (2 character) standard. However, the `WC_Tests_Order_Functions::test_wc_get_order_address_params()` test method was using 'USA' instead of 'US'.