test: Rework testing of maildir-synchronization feature.

test: Rework testing of maildir-synchronization feature.

This change reworks these tests in several ways:

1. Bring tests into "new" test style preferring test_expect_equal over
   test_expect_success in almost all cases.

2. Don't emit test results for intermediate items not actually being
   tested, (things like "no new messages", "search for message",
   etc.). Those things are already covered by existing tests such as
   "basic" or "search" and only serve to obscure what's actually being
   tested.

3. Change sense of the test showing failure to rename a file from
   "new" to "cur" when "cur" doesn't exist.

   In this case, notmuch should detect that this is not a maildir and
   should not attempt to do any renaming of the file.

4. Extend dump/restore test to also exercise addition of tag, not just
   removal.

Both items #3 and #4 above show shortcomings in the current
implementation. These are currently resulting in test results of FAIL
and indicate bugs that need to be fixed.