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/diary/delete_review.rs

@@ -52,7 +52,3 @@ pub async fn execute(deps: &DeleteReviewDeps, cmd: DeleteReviewCommand) -> Resul
 
     Ok(())
 }
-
-#[cfg(test)]
-#[path = "tests/delete_review.rs"]
-mod tests;