simple-test: fix awk syntax for unmount only by fstype

But we could just use 'umount -t' I guess.
simple-test: change dir to root before unmount, to avoid opened fds
simple-test: use 'umount -t /path/to/mnt' to avoid umounting extra mnts
simple-test: call umount from cleanup() only when mount point exists

this will avoid errors when cleanup() called before doing actuall job,
IOW just after make and before all tests.