Fix WebAuthnWebdriverTests

[Kehrlann]

WebAuthnWebdriverTests.loginWhenAuthenticatorRegisteredThenSuccess was flakey, and failed with StaleElementReferenceException.

This is a race condition between:

• asynchronously changing the page location through javascript on registration success, using window.location.href
• asserting that there is a success message displayed, by calling WebElement alert = this.driver.findElement(...) and then calling alert.isDisplayed(), where the alert could reference an element on the page BEFORE the redirect

Fix: ensure the page change is complete before asserting on elements in the page, with an additional assertion.

reviewer: @rwinch