Core tests open their store through local::memory_conn

Seven test modules each built a migrated in-memory store by hand: open, migrate,
and (in sharing) wrap it in a Db. local::memory_conn() is that, and
open_in_memory uses it too. Each module's db() is now one line, and the schema,
Connection and Mutex imports it needed are gone.

DRY pass #2, batch 2, F9 (#5372).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/src/local/store.rs

@@ -1143,12 +1143,9 @@ mod tests {
     //! against a real schema catches, which is what every test here runs.
 
     use super::*;
-    use crate::local::schema;
 
     fn db() -> Connection {
-        let conn = Connection::open_in_memory().expect("in-memory db");
-        schema::migrate(&conn).expect("migrate");
-        conn
+        crate::local::memory_conn().expect("in-memory db")
     }
 
     fn note(conn: &Connection, body: &str) -> Note {