Test static validator set changes in Aura

[HCastano]

As part of fixing #10306 through #10467 I figured it would be good to write a test to simulate a change in validators that were defined in a chain spec. This turned out to be a bit harder than expected so I plan to tackle it separately. The idea would be to create a new aura_test_multi_validators.json chain spec that has something like

"multi":
{
    "0": {
         "list": ["<address0>"]
    },
     "10": {
         "list": ["<address0>, <address1>"],
     }
}

and check the validator list at block 0. After making some fake blocks we should then jump to block transition_block + 1 and check the validator list again. At this point we should expect the new validator to be part of this list.

[adria0]

Closing issue due to its stale state.