test: ums: Add sleep before unmount directory

This change helps to run script on machines with quite long uptime.
Without this the following error emerges:

File: ./dat_14M.img
umount: /mnt/tmp-ums-test: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
TX: md5sum:083d3d22b542d3ecba61b12d17e03f9f
mount: /dev/sdd6 already mounted or /mnt/tmp-ums-test busy
mount: according to mtab, /dev/sdd6 is already mounted on /mnt/tmp-ums-test

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>