Add tests covering remaining plugin functionality

[elPixelador]

Extends the PHPUnit suite from the order filter tests (#23) to cover the rest of the plugin.

• Bootstrap: more WordPress/WooCommerce stand-ins (users, products, cart, scripts, escaping), a queue for fake Spiff responses, and hook state reset between tests.
• New tests: activation notification, admin menu/settings/product fields, product list and product page buttons, script enqueueing, adding designs to the cart, cart prices, customer selections and preview images in cart, checkout order item meta, customer portal shortcode.
• Replaced the dummy RequestsTest with tests for request headers and region URL selection.
• Extended CreateOrderTest with customer/address data, US routing and failed request handling.

Known bugs found while writing these (not covered here, to be fixed separately with tests):

• spiff_show_metadata_in_cart reads spiff_exported_data without checking it exists
• spiff_get_transaction interpolates the transaction ID into the query and doesn't handle missing transactions
• spiff_get_woo_id_from_transaction logs an undefined variable
• spiff_replace_default_button_on_product_list fatals on buttons that aren't valid XML
• spiff_enqueue_ecommerce_client's ?? "AU" default never applies

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