add a test script that backdoors `ls`

I didn't use `which ls` because that doesn't actually work on my system and it might not work on others. I have a shell script that wraps ls and `which ls` returns the path to a shell script. That might be the case for others too, so I just used /bin/ls. Let me know if that's a problem on some systems.
Also let the user know that backdoor_ls is available for testing.