The map the devs use to test is still in the game.
To get to it, you can use the command:
start pc-basictesting_dg
If you want it on a public server, use the command on your server, or if you want it for "singleplayer" then use it on the main menu.
If it doesn't work, you entered it wrong!

In the map, there are "npcs" that just stand there, and can't do anything. They are great for getting acheivements like Silent P, or Timber.
It's also good for testing out skins.
You can kill "npc" teammates with most attacks on the map.
Have fun... doing whatever you want to do on this secret map!