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/tests/integrations/cleanup_test.rs

@@ -0,0 +1,12 @@
+use domain::testing::InMemoryStore;
+
+use application::integrations::cleanup;
+
+#[tokio::test]
+async fn returns_zero_when_nothing_to_clean() {
+    let watch_events = InMemoryStore::new();
+
+    let count = cleanup::execute(watch_events).await.unwrap();
+
+    assert_eq!(count, 0);
+}