P-chain UTs consolidation 1 - Genesis

[abi87]

Why this should be merged

This is the first of three PRs which simplify UT creation in P-chain. This PR centralize genesis creation in UTs. Genesis creation is moved to a genesistest package and kills 200 LOC without giving up any functionality.

Thanks @joshua-kim and @marun for the discussions that brought me to these changes.

How this works

Centralize genesis build in UTs to avoid a ton of code duplication.

How this was tested

CI

[github-actions[bot]]

This PR has become stale because it has been open for 30 days with no activity. Adding the lifecycle/frozen label will cause this PR to ignore lifecycle events.