The commit moves, test_create_mirror_user_despite_race which is not related
to message sending from MessagePOSTTest class in test_message_send.py to
test_mirror_users.py.
Starting with extracting out MirroredMessageUsersTests as it is related to
mirror users than anything message-specific.
In a future commit, may extract out some tests from MessagePOSTTest as well
but still deciding on those.
Anders Kaseorg
Mohit Gupta