DRY pass #3: core tests open their store and seed a label one way (#5373)

Five test modules wrote the same db() over memory_conn, and portable.rs
unwrapped it inline; local::test_db() is that. Four pull/push tests wrote the
same label INSERT; local::seed_label(conn, id, name, dirty) is it. Both are
cfg(test), beside memory_conn, as wire::sample_note is beside wire::Note.

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

core/src/local/store.rs

@@ -1123,9 +1123,7 @@ mod tests {
 
     use super::*;
 
-    fn db() -> Connection {
-        crate::local::memory_conn().expect("in-memory db")
-    }
+    use crate::local::test_db as db;
 
     fn note(conn: &Connection, body: &str) -> Note {
         create_note(