This commit moves InternalPrepTest test class to test_message_send.py
because it tests internal_send_* and internal_prep_* functions which are
used for internal message sending in zulip.
This commit moves few tests related to testing proper sending of private
messages from PrivateMessagesTest class in test_messages.py to a new class
in test_message_send.py.
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.
This commit extracts out MessagePOSTTest class from test_messages.py
intially.
In future commits other related message sending tests will be moved from
test_messages.py to test_message_send.py.
Mohit Gupta