Update fillCreditCard selector to use __privateStripeFrame preffix (#3854)

Author: diegocurbelo

tests/e2e/utils/payments.js

@@ -128,15 +128,21 @@ export async function fillCreditCardDetailsShortcode( page, card ) {
  */
 export async function fillCreditCardDetailsLegacy( page, card ) {
 	await page
-		.frameLocator( '#wc-stripe-card-number-element iframe' )
+		.frameLocator(
+			'#wc-stripe-card-number-element iframe[name^="__privateStripeFrame"]'
+		)
 		.locator( 'input[name="cardnumber"]' )
 		.fill( card.number );
 	await page
-		.frameLocator( '#wc-stripe-card-expiry-element iframe' )
+		.frameLocator(
+			'#wc-stripe-card-expiry-element iframe[name^="__privateStripeFrame"]'
+		)
 		.locator( 'input[name="exp-date"]' )
 		.fill( card.expires.month + card.expires.year );
 	await page
-		.frameLocator( '#wc-stripe-card-code-element iframe' )
+		.frameLocator(
+			'#wc-stripe-card-code-element iframe[name^="__privateStripeFrame"]'
+		)
 		.locator( 'input[name="cvc"]' )
 		.fill( card.cvc );
 }