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/watchlist.rs

@@ -0,0 +1,12 @@
+mod support;
+
+#[path = "watchlist/add_test.rs"]
+mod add_test;
+#[path = "watchlist/get_test.rs"]
+mod get_test;
+#[path = "watchlist/get_watchlist_for_owner_test.rs"]
+mod get_watchlist_for_owner_test;
+#[path = "watchlist/is_on_test.rs"]
+mod is_on_test;
+#[path = "watchlist/remove_test.rs"]
+mod remove_test;