Reduce duplicated code in miner tests

[LesnyRumcajs]

Miner tests effort was done by several devs in parallel, sometimes resulting in duplicated code / re-defined constants etc. For example, there are 13 occurrences of const BIG_BALANCE: u128 = 1_000_000_000_000_000_000_000_000u128;. It would be nice to go through those different files and extract common constants and helper methods.

It may be worth checking out other actors as well. On a higher level, it may be worth extracting some code to test_utils if multiple actor tests need a similar piece of technology, e.g. bitfield creation/manipulation.