rcll-central: add dummy machines for challenges

[snoato]

Tightening the conditions for goal executability in the central agent to guarantee proper behavior in a normal game can lead to non-executability in challenges. One example is MOUNT-RING goals where the proper condition requires two RS. However, in challenges only one might be available. Instead if making the condition more complex, one approach to solve this is to add dummy machines where needed, so that the conditions are satisfied without modifications.