For testing https://github.com/Automattic/woocommerce-payments/pull/5346

wcpay-test-pr-5346.php

<?php
// Prevent maybe_process_upe_redirect. For testing UPE flow.
// Stop at this point https://github.com/Automattic/woocommerce-payments/blob/41efc3b23fe952fefdb646eb6980cf8faa90489f/includes/payment-methods/class-upe-payment-gateway.php#L591
add_action(
	'check_admin_referer',
	function( $action ) {
		if ( 'wcpay_process_redirect_order_nonce' === $action ) {
			exit('Prevent maybe_process_upe_redirect');
		}
	}
);


// For testing non-UPE flow - 3DS card
// Stop code execution at this point https://github.com/Automattic/woocommerce-payments/blob/41efc3b23fe952fefdb646eb6980cf8faa90489f/includes/class-wc-payment-gateway-wcpay.php#L2492-L2492
add_action(
	'check_ajax_referer',
	function( $action ) {
		if ( 'wcpay_update_order_status_nonce' === $action ) {
			exit('Prevent update_order_status');
		}
	}
);

// Prevent processing the webhook endpoint
add_filter( 'rest_pre_dispatch',
	function ( $response, $wp_rest_server, $request ) {
		if( '/wc/v3/payments/webhook' === $request->get_route() ) {
			$response = new WP_Error(
				503,
				'Prevent the webhook endpoint for testing WCPay'
			);
		}

		return $response;
	},
	10,
	3
);