Make max default block gas parametrizable in `fill` command (currently ~30 million)

[marioevz]

Regarding the gas increment that is being pushed for Osaka, I think we should make the max-gas of our tests configurable.

We hard-code 30 million gas in many places because we assumed that this is the limit, but I think we should make it a parameter to fill with a default number, and we increment this number every time we want to increment the mainnet defaults to generate all tests with an increased gas automatically.