test: avoid race in sender ip cleanup imap check

[ccclxxiii]

pr overview:

remove the wait_for_incoming_msg() call from test_hide_senders_ip_address and poll imap directly before delta chat consumes or moves the message.

this may fix the ci failure where:

assert msgs, "expected at least one message"

fails after the message has already been processed by the client.

changes

• open imap connection before sending the message
• remove user2.wait_for_incoming_msg()
• poll mailbox.fetch(mark_seen=False) with a short timeout
• preserve the existing sender ip cleanup assertion

notes

this is intended as a ci stabilization change for integration testing and should not affect runtime behavior.

[ccclxxiii]

closing while we wait on chatmail/cmlxc#19