QA: Revisit the login page when attempt to retry login

What does this MR do?

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21858 I added a retry loop but I assumed the option to sign out was available (as it appeared to be). This MR revisits the URL of the sign in page directly instead of trying to use the UI.

When the User factory attempts to create a new user, registration fails intermittently - the user is created but isn't signed in - it looks like the UI has a sign-out option, but it doesn't. See https://gitlab.com/gitlab-org/gitlab-ce/issues/51316 and in particular https://gitlab.com/gitlab-org/gitlab-ce/issues/51316#note_105880780

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides