db test: Add missing after.close call

As we add more tests for future migrations, Drift will start complaining
about opened databases that are not closed.  Always remember doing this
will ensure that we don't leak states to other database tests.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

test/model/database_test.dart

@@ -130,6 +130,7 @@ void main() {
         ...accountV1.toJson(),
         'ackedPushToken': null,
       });
+      await after.close();
     });
   });
 }