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/sync/sharing.rs

@@ -85,15 +85,10 @@ pub async fn unshare(
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::local::schema;
     use crate::sync::client::Member;
-    use rusqlite::Connection;
-    use std::sync::Mutex;
 
     fn db() -> Db {
-        let conn = Connection::open_in_memory().expect("in-memory db");
-        schema::migrate(&conn).expect("migrate");
-        Db(Mutex::new(conn))
+        crate::local::open_in_memory().expect("in-memory db")
     }
 
     #[test]