[tests] reduce flakiness in test_incoming_mail
[tests] reduce flakiness in test_incoming_mail

It was adding messages with notify_just_mdoc set to True (default).
leap.mail suggests that this flag is set to True for performance, but it
is unsafe.
I was getting a lot of false negatives on tests, now it is not flaky
anymore.
-- Issue #445

pixelated-project/pixelated-user-agent,pixelated/pixelated-user-agent,sw00/pixelated-user-agent,pixelated-project/pixelated-user-agent,SamuelToh/pixelated-user-agent,SamuelToh/pixelated-user-agent,torquemad/pixelated-user-agent,pixelated/pixelated-user-agent,torquemad/pixelated-user-agent,sw00/pixelated-user-agent,kaeff/pixelated-user-agent,PuZZleDucK/pixelated-user-agent,pixelated/pixelated-user-agent,phazel/pixelated-user-agent,rdoh/pixelated-user-agent,pixelated-project/pixelated-user-agent,SamuelToh/pixelated-user-agent,rdoh/pixelated-user-agent,rdoh/pixelated-user-agent,PuZZleDucK/pixelated-user-agent,SamuelToh/pixelated-user-agent,phazel/pixelated-user-agent,pixelated-project/pixelated-user-agent,kaeff/pixelated-user-agent,pixelated/pixelated-user-agent,sw00/pixelated-user-agent,phazel/pixelated-user-agent,kaeff/pixelated-user-agent,pixelated/pixelated-user-agent,phazel/pixelated-user-agent,rdoh/pixelated-user-agent,PuZZleDucK/pixelated-user-agent,sw00/pixelated-user-agent,SamuelToh/pixelated-user-agent,torquemad/pixelated-user-agent,sw00/pixelated-user-agent,torquemad/pixelated-user-agent,phazel/pixelated-user-agent,kaeff/pixelated-user-agent,pixelated-project/pixelated-user-agent,torquemad/pixelated-user-agent,rdoh/pixelated-user-agent,PuZZleDucK/pixelated-user-agent,kaeff/pixelated-user-agent,PuZZleDucK/pixelated-user-agent