Sandboxed Payment Gateway for WooCommerce
Official: woocommerce/woocommerce-gateway-dummy: A Dummy Payments gateway for your WooCommerce website.
WooCommerce Order Test Sandbox Payment Gateway for WooCommerce Fake Pay For WooCommerce Dummy Gateway for WooCommerce WooFakePay
Testing WooCommerce payments:
WooPayments - WooCommerce free plugin has test mode