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.
Add link to build environment (github.com/azat/simplefs-bld)