Add tests about the clone function

[RexSkz]

I just found a tool that can tell whether two files share the same physical blocks: https://github.com/dyorgio/apfs-clone-checker

$ gcc clone_checker.c -o clone_checker

$ chmod +x clone_checker

$ ls
LICENSE
README.md
clone_checker
clone_checker.c

# normal copy
$ cp README.md README.copy.md

# use copyfile
$ cp -c README.md README.clone.md

$ ./clone_checker README.md README.copy.md
0

$ ./clone_checker README.md README.clone.md
1

# make some changes
$ vi README.clone.md

$ ./clone_checker README.md README.clone.md
0

It could be used to test the clone function.