Check for possible edge case in `bcpc_query()`

@jranke fixed the way `bcpc_query()` parses activities and sub-activities in #348, #349 (thank you!).  I think the fix relies on their always being one sub-activity for every activity, so we should look for any cases where activities have multiple sub-activities and add a test.  

It may be best to just re-work the output to be a tibble (something that might be good to do anyways, #218).