test: Rework recently-added additional maildir-sync tests

These needed to be changed to be brought up to the current state of
the maildir-sync tests. This includes style changes, but also the
elimination of any assumption about pre-existing message filenames,
(such as msg-003) which actually don't exist anymore.

Also, the known broken tests are changed to emit FAIL rather than
BROKEN simply to make them easier to fix, (so that they print the
current problems rather than hiding them).

Finally, an additional test is added to ensure that when a duplicate
file is added without flags, it doesn't invalidate flags from other
duplicates, (instead the flags are effectively merged).