btrfs-progs: tests: umount TEST_MNT in clean-tests.sh

btrfs-progs: tests: umount TEST_MNT in clean-tests.sh

If a testcase failed, we can't run it(or other tests needs mount) again,
  # ./misc-tests.sh 007
   [TEST]   007-subvolume-sync
   failed: fail
   test failed for case 007-subvolume-sync
  # ./misc-tests.sh 007
   [TEST]   007-subvolume-sync
   failed: mount /root/btrfs-progs/tests/test.img /root/btrfs-progs/tests/mnt
   test failed for case 007-subvolume-sync

This patch add "umount $TEST_MNT" to clean-tests.sh, to let user
clean mountpoint easily.

After patch:
  # ./misc-tests.sh  007
   [TEST]   007-subvolume-sync
   failed: fail
   test failed for case 007-subvolume-sync
  #
  # clean-tests.sh
  #
  # ./misc-tests.sh  007
   [TEST]   007-subvolume-sync
   failed: fail
   test failed for case 007-subvolume-sync

Suggested-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
[added sudo helper to umount]
Signed-off-by: David Sterba <dsterba@suse.com>