test: Use increment_mtime rather than sleep

The sleep was to force the directory's mtime to advance between the
previous notmuch new and the subsequent rm;notmuch new.

The current convention is to use the existing increment_mtime function
for this purpose, (which avoids the test suite being slowed down by
calls to sleep).

Thanks to Austin Clements for noticing this undesired sleep.
(cherry picked from commit 55a78d5dbd96b80939458d9d6cc8a536b1befa87)