Use selector to indicate Tumblr is ready for login (#98841)

packages/calypso-e2e/src/lib/pages/marketing-page.ts

@@ -168,9 +168,8 @@ export class MarketingPage {
 	 * @param {string} param1.password Tumblr password.
 	 */
 	async setupTumblr( popup: Page, { username, password }: { username: string; password: string } ) {
-		// Wait for the page load to complete. Otherwise, a `Cannot POST /login` error
-		// is shown.
-		await popup.waitForLoadState( 'networkidle' );
+		// This selector allows us to proceed without waiting for `networkidle`.
+		await popup.waitForSelector( '[data-tumblr-ready="true"]' );
 
 		// Fill in the email and password.
 		await popup.getByRole( 'textbox', { name: 'email' } ).fill( username );