P-chain UTs consolidation 2 - Context and Config

[abi87]

Why this should be merged

In https://github.com/ava-labs/avalanchego/pull/2107 we moved to a genesistest the machinery to build UTs genesis. Here we keep on going with vm.Config and vm.Ctx, moving them to a configtest package Thanks @joshua-kim and @marun for the discussions that brought me to these changes.

How this works

Just refactor UTs setup

How this was tested

CI

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