[16.0][FIX] mail_activity_done: fix test failing between 22:00 and 00:00 utc

[huguesdk]

date.today() returns the day in utc, while mail.activity.mixin.activity_state's "today" value means the current date in the user's timezone. this made a test fail between 22:00 and 00:00 utc (when brussels uses the cest timezone (utc+2), otherwise between 23:00 and 00:00 utc when it uses the cet timezone (utc+1)).

a second commit improves the test code somewhat.