pytest: Use --lazy with umount

Sometimes when doing tests on real hardware we sometimes run in to the
case where some of these mounts haven't been fully flushed.  Using the
--lazy option with umount will allow us to continue while letting the OS
handle flushing the data out still.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>