squash? Use a better alternative to accessing the delegates

This proposes an alternative way to set the schema version
to the parent commit, by utilizing some more implementation details.

References:
  https://github.com/simolus3/drift/blob/18cede15/drift/lib/src/runtime/executor/helpers/engines.dart#L459-L495
  https://github.com/simolus3/drift/blob/18cede15/drift/lib/src/sqlite3/database.dart#L198-L211
  https://github.com/simolus3/sqlite3.dart/blob/4de46afd/sqlite3/lib/src/implementation/database.dart#L69-L85

Signed-off-by: Zixuan James Li <zixuan@zulip.com>

test/model/database_test.dart

@@ -111,21 +111,15 @@ void main() {
       await before.customStatement('ALTER TABLE accounts ADD extra_column int');
       await check(verifier.migrateAndValidate(
         before, 2, validateDropped: true)).throws<SchemaMismatch>();
+      // Override the schema version.
+      // TODO(upstream): Expose a better interface for testing this.
+      await before.customStatement('PRAGMA user_version = 999;');
       await before.close();
 
       // We need a new connection because migrations are only run when the
       // database is first opened (see [DelegatedDatabase.ensureOpen]), and
       // running custom statements opens it.
       final after = AppDatabase(schema.newConnection());
-      // Trick the drift into believing that the schema version is
-      // higher than it actually is, so that a downgrade migration is triggered.
-      // This relies on some relevant implementation details:
-      // - [DelegatedDatabase._runMigrations]
-      // - [VerifierImplementation.migrateAndValidate]
-      // TODO(upstream): Expose a better interface for testing this.
-      await ((after.executor as DelegatedDatabase)
-        .delegate.versionDelegate as DynamicVersionDelegate).setSchemaVersion(999);
-
       await verifier.migrateAndValidate(after, 2, validateDropped: true);
       await after.close();
     });