Test draft source and edit_message_mode

This tests adding and removing messages to the draft source, as well as the relevant methods from edit_message_mode.

The tests do not correctly remove all remnants of the files in /tmp.. don't know what is going on there, but if someone wants to have a go please go ahead. Whether it is still a good idea to include the tests like this needs to be reviewed.

Writing and loading draft messages are ensured to be done in utf-8 now, hopefully fixes (and tests) issue #340. Issue #343, still remains, but the tests should hopefully be possible to extend to include this test.