test: move domain tests to tests/

Self dev-dep w/ test-helpers for domain::testing. Also moved social blanket-impl + InMemoryStore tests (public API only). No private-item exceptions.

crates/domain/src/events.rs

@@ -168,7 +168,3 @@ impl EventEnvelope {
         self.ack.nack().await
     }
 }
-
-#[cfg(test)]
-#[path = "tests/events.rs"]
-mod tests;