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/worker.rs

@@ -84,7 +84,3 @@ async fn dispatch(handlers: Arc<Vec<Arc<dyn EventHandler>>>, envelope: EventEnve
         tracing::error!("ack failed: {e}");
     }
 }
-
-#[cfg(test)]
-#[path = "tests/worker.rs"]
-mod tests;