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/portable.rs

@@ -595,7 +595,7 @@ mod tests {
     use super::*;
 
     fn store() -> (Connection, BlobStore, std::path::PathBuf) {
-        let conn = crate::local::memory_conn().unwrap();
+        let conn = crate::local::test_db();
         let blobs = crate::sync::blobs::scratch("portable");
         let dir = blobs.root().to_path_buf();
         (conn, blobs, dir)