notmuch-test: Eliminate sleeps to speed up test suite run

notmuch-test: Eliminate sleeps to speed up test suite run

We were sleeping merely to ensure that our updates to the mail store
would result in the mtime of the appropriate directories being
updated. We make the test suite run faster by not sleeping, but
instead explicitly updating the mtime of the directory to a future
time with touch.

We're careful to ensure that the time is not merely in the future
compared to the current time, but also later than any previous update
to the same directory mtime.