tests: pull ComputeBlockVersion test into its own function
tests: pull ComputeBlockVersion test into its own function

The intent here is to allow checking ComputeBlockVersion behaviour with
each deployment, rather than only testdummy on mainnet. This commit does
the trivial refactoring component of that change.

Github-Pull: #21377
Rebased-From: 63879f0a4760c0c0f784029849cb5d21ee088abb

litecoin-project/litecoin,qtumproject/qtum,rnicoll/dogecoin,qtumproject/qtum,litecoin-project/litecoin,rnicoll/dogecoin,qtumproject/qtum,rnicoll/dogecoin,litecoin-project/litecoin,litecoin-project/litecoin,rnicoll/dogecoin,litecoin-project/litecoin,qtumproject/qtum,rnicoll/dogecoin,litecoin-project/litecoin,qtumproject/qtum,qtumproject/qtum,qtumproject/qtum