Remove loadHTMLFixture usage in `spec/frontend/authentication/`

Description

This issue is part of Replace loadHTMLFixture with static imports (&10244 - closed). Let's replace references to loadHTMLFixture in the following places:

• spec/frontend/authentication/webauthn/authenticate_spec.js:38: loadHTMLFixture('webauthn/authenticate.html');
• spec/frontend/authentication/webauthn/register_spec.js:28: loadHTMLFixture('webauthn/register.html');

Implementation Guide

Please see the parent epic for reasoning and examples of how to do this. If you'd like to pick up this issue, please leave a comment so that others do not pick it up :)