selftest/reproducible: Don't call sync between each file compare

Calling sync between each file compare is horrible performance wise
as we compare thousands of files. We don't care about IO latency here
so disable.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 25f78abd)
Signed-off-by: Steve Sakoman <steve@sakoman.com>