Add acceptance tests for WooCommerce integration

[johnbillion]

User Switching provides specific integrations with WooCommece, bbPress, and BuddyPress, but these integrations aren't covered by tests (because it's a pain to set up).

Acceptance tests should be added for all of these. Let's start with WooCommerce.

This will help when issues are reported such as https://wordpress.org/support/topic/possible-problem-with-empty-carts-in-woocommerce-10-1/ which it's not yet clear if this is specific to switching users or a general bug with multiple user accounts in WooCommerce 10.1.