[QA] Reload page to check if branch is deleted

What does this MR do?

When the "Delete merged branches" button is used the UI doesn't update automatically. So when the page is refreshed it's possible that the branch will still be present.

This checks for the branch to be gone and reloads if it is not, repeating until the default timeout (60s).

See: https://gitlab.com/gitlab-org/quality/nightly/issues/62

Does this MR meet the acceptance criteria?

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