Fix basic login test stability

What does this MR do?

Fixes basic login test stability by waiting for the login tab after sign out.

Closes gitlab-org/quality/staging#57 (closed)

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content