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/users/resolve_username_to_id_test.rs

@@ -0,0 +1,20 @@
+use domain::value_objects::Username;
+
+use crate::support::TestContextBuilder;
+use application::users::deps::ResolveUsernameDeps;
+use application::users::resolve_username_to_id;
+
+#[tokio::test]
+async fn resolve_username_to_id_returns_none_for_unknown() {
+    let b = TestContextBuilder::new();
+    let deps = ResolveUsernameDeps {
+        user: b.user_repo.clone(),
+    };
+    let unknown = Username::new("nosuchuser".into()).unwrap();
+    assert!(
+        resolve_username_to_id::execute(&deps, &unknown)
+            .await
+            .unwrap()
+            .is_none()
+    );
+}