isolate tested gateway within its test to avoid dependencies that lea…

…d to test regressions

tests/unit/test-class-wc-payment-gateway-wcpay.php

@@ -236,6 +236,9 @@ public function set_up() {
 			->getMock();
 
 		$this->init_gateways();
+
+		// Replace the main class's gateway for testing purposes.
+		$this->_gateway = WC_Payments::get_gateway();
 		WC_Payments::set_gateway( $this->card_gateway );
 
 		$this->woopay_utilities = new WooPay_Utilities();
@@ -266,6 +269,9 @@ public function tear_down() {
 		// Restore the cache service in the main class.
 		WC_Payments::set_database_cache( $this->_cache );
 
+		// Restore the gateway in the main class.
+		WC_Payments::set_gateway( $this->_gateway );
+
 		// Fall back to an US store.
 		update_option( 'woocommerce_store_postcode', '94110' );
 		$this->card_gateway->update_option( 'saved_cards', 'yes' );