Wait for username validation JS to complete

What does this MR do?

Fixes qa/specs/features/browser_ui/1_manage/login/register_spec.rb spec instability by waiting for username validation JS to complete.

Related issue: #297468 (comment 499404026)

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content