testCMFActivity: Do not hide actual failure cause.

The main change here is the reduction of try..except scope, so failing the
first assertion does not cause an AttributeError about original_query.
Also, avoid code duplication by just checking final number of messages
outside the try..except block.
Also, improve coding style (spaces around operators and commas).
Also, avoid single-use variable.
Also, fix typo in comment.