When setting the listeners in the IMAP Folder, we avoid setting more
than one listener for the same imap mailbox (because in some situations
we were registering way too many listeners).

this was making the pixelated inbox registering the notification and
therefore the imap mailbox not being registered.

this MR also refactors the way pixelated is initialized, so that it
avoid creating a second Account instance. In this way, we make sure that
the pixelated mua and the imap server share the same collections for a
given mailbox, and therefore any of the two is able to get a
notification whenever the other adds a message to the mailbox.

- Resolves: #8846, #8798

in the variant that was merged, we call a failure method to get the
exception, so it's better to test a real failure initialized with an
exception.

besides, I'm starting to hate mocks.

This commit originall contained a fix for a wrong variable name that
avoided the outgoing service to be launched. This had been fixed in a
concurrent mr that got merged before this one. However, this MR
contained a couple of tests for the OutgoingService, and I'm merging
those tests now.