When visiting a user profile, and then opening the search, there's an option to filter down by posts made by that user.
When clicking that option, it used to pre-fill the "search bar" with "@<username>" to filter down the search.
This restore this behaviour and add a system spec to ensure it doesn't regress.
Context - https://meta.discourse.org/t/in-posts-by-search-option-does-not-work-when-clicked/312916
