Modify "testnet scenarios" to make it automatic

The current implementation of the testnet scenarios require that:

1. a snapshot is specified, that the scenario downloads and imports, in order to speed up bootstrap
2. that the user manually funds the two operator accounts generated by the scenario using a testnet faucet.

In the context of TeCI,

1. doesn't make much sense since the chain is basically empty before the generator runs. Therefore, the easiest thing is just to make snapshot import optional.

For 2., it seems the easiest thing is to modify the scenario so that instead of waiting for users to fund the operator accounts, the scenario can be provided with a prefunded account from which it can transfer the funds to the two operator accounts. In TeCI, the normal bootstrap accounts are available for this purpose.