test: move activitypub tests to tests/

Assert via serialized JSON (fields pub(crate)); normalize_hashtag covered via review tags. No inline exceptions.

crates/adapters/activitypub/tests/objects.rs

@@ -0,0 +1,4 @@
+#[path = "objects/review_object_test.rs"]
+mod review_object_test;
+#[path = "objects/watchlist_object_test.rs"]
+mod watchlist_object_test;