Less ambiguous name for test_events.py
Remove duplicate lower() call.

We already make sure all members of existing_labels and new_labels are
lowercase.
Canonicalize All Mail to be 'archive' in our datastore
Update test dump for new ImapAccount columns
Canonicalize folder_names keys as lowercase

As a part of this, I'm also simplifying the logic in Gmail's
folder_names() detection to get rid of the janky usage of
boolean flags.