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/support/mod.rs

@@ -0,0 +1,10 @@
+//! Shared fakes and builders for the application integration tests.
+//! Included via `mod support;` from each `tests/<module>.rs`; not every
+//! binary uses every helper.
+#![allow(dead_code, unused_imports)]
+
+mod commands;
+mod context;
+
+pub use commands::*;
+pub use context::*;