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/tests/testing.rs

@@ -0,0 +1,2 @@
+#[path = "testing/in_memory_store_test.rs"]
+mod in_memory_store_test;