I like making command block contraptions. Occasionally, I make a contraption that I want to use on a multiplayer server (or offer to server operators), but I don't know of a way to test that it'll actually work.
I don't run a server that I can test on, nor do I want to have to get friends/family on this non-existent server in order to do tests and debugging. I also don't have op privileges on another server where I could do these tests (nor would I want to since it could disrupt the server). Finally, I only own one Minecraft account, and I'm not really interested in buying a second account just for the purposes of testing command blocks.
So given that, what's the best way for me to perform my testing and debugging? Is there a simple (and legitimate) way for me to add one or more players that I can control to my otherwise single player command block world?