parent::setUp() should be called inside test setup methods to make sure database changes are automatically reverted after each test is executed. This way it is not necessary to manually remove inserted data.
The cache busting currently in `wc_add_order_item_meta`, `wc_update_order_item_meta`, and `wc_delete_order_item_meta` doesn't actually bust anything. The cache line looks like it is from 2.6. The relevent cache to bust is actually in the `order-items` group and has a different key/prefix.
This bug allows your meta to get out of sync if you use these functions and then try to access a value from a CRUD object.
You can see this in the `test_wc_order_item_meta_functions` test I've added. If you keep your `wc-order-item-functions.php` as is, the asserts against `$item->get_meta` will fail.
To test:
* `phpunit --filter=test_wc_order_item_meta_functions`.
* Try before applying the `wc-order-item-functions.php` changes and after.