test: move application tests to tests/

test_support/test_helpers -> tests/support; watched_at unified w/ domain test_watched_at (2024-03-15).
Kept inline: users::diary_filter tests (feed_sort_to_direction_all_variants, paged_user_filter_builds_correctly) - pub(super) helpers in private module.
Dropped stale never-compiled users/tests/get_users.rs (old execute signature).

crates/application/src/users/resolve_username_to_id.rs

@@ -17,7 +17,3 @@ pub async fn execute(
     let user = deps.user.find_by_username(username).await?;
     Ok(user.map(|u| u.id().clone()))
 }
-
-#[cfg(test)]
-#[path = "tests/resolve_username_to_id.rs"]
-mod tests;