test: Use increment_mtime rather than sleep

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.