Created testing instructions.

Author: wavvves

docs/internal-developers/testing/releases/871.md

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+# Testing notes and ZIP for release 8.7.1
+
+Zip file for testing: [woocommerce-gutenberg-products-block.zip](https://github.com/woocommerce/woocommerce-blocks/files/9766268/woocommerce-gutenberg-products-block.zip)
+
+## Feature plugin and package inclusion in WooCommerce
+
+### Fixed an issue where JavaScript errors would occur when more than one extension tried to filter specific payment methods in the Cart and Checkout blocks ([7377](https://github.com/woocommerce/woocommerce-blocks/pull/7377))
+
+#### User Facing Testing
+
+1. Install and activate [some-extension-name.zip](https://github.com/woocommerce/woocommerce-blocks/files/9764404/some-extension-name.zip) and [WooCommerce Conditional Shipping and Payments](https://woocommerce.com/products/conditional-shipping-and-payments/).
+2. Activate the Cash on Delivery payment method.
+3. Go to the Cart block and view the JS console. Ensure you don't see any errors such as `Error when executing callback for cheque in some-extension-name TypeError: namespacedCallbacks[namespace] is not a function`.
+4. Go to the Checkout block and check that no similar errors appear.
+5. Change your city to `Denver` and ensure the Cash on Delivery method is not available.