Creating Mining Competition on regtest
Previous versions of bitcoin core included a miner, and you could use the RPC call setgenerate to activate it. However, this functionallity was removed on Bitcoin Core v0.13.0. In any case, this does does not seem to work on regtest (I have tried it with v0.11.1 and it returns a message saying to use generate instead).

Whenever I need to "simulate" nodes competing to mine in a regtest network, I define their mining power and randomly select which one of them is going to mine taking into account their defined mining power. Then, I send a generate 1 RPC call to the miner that has been selected.