Add churn tests for when churn limit scales with v-set size

[djrtwo]

We were missing tests for the churn limit when it begin to scales at the v-set size -- that is, when len(active_validators) >= CHURN_LIMIT_QUOTIENT * (1 + MIN_PER_VALIDATOR_CHURN_LIMIT). This doesn't happen on mainnet config until 327680 validators but better to get it in now than later!

To achieve this in a "minimal" way for our minimal config it required adjusting CHURN_LIMIT_QUOTIENT down from 65536 to 32. This allows for this scaled case to be triggered at 160 validators instead of ~300k. Although this would break any minimal testnets, as far as I know we do not have any persistent minimal testnets today and thus can safely modify this config for testing purposes.

This PR involves duplicating existing queue churn limit tests so that one test runs for min-churn-limit and one runs for scaled-churn-limit. get_validator_churn_limit is called within process_registry_updates and initiate_validator_exit so we identified tests in test_process_registry_updates and test_voluntary_exit for this scaled-churn-limit extension.

[ralexstokes]

looks great and adds expanded test coverage!!

are you concerned about testing the scenario where the exit queue spans multiple epochs? the exit tests only consider "spill-over" into 1 epoch at the end of the exit queue -- we can imagine spilling over into several epochs after the end of the exit queue depending on the number of exits

i do see the "multi-epoch" case handled w/ forced ejections due to low balance so that may suffice for now...