Suggestion for "feat(regtest): Add regtest halving interval and port test"

[arya2]

This is a suggestion PR for #8888 that:

• Adds a height_for_halving_index() function to be used for calculating the height for first index on custom Testnets and Regtest (though it's not needed on Regtest as the relevant parameters are not configurable and a hard-coded value is fine).
• Factors calculating of the number of halvings out of halving_divisor() into num_halvings() for use in tests

@oxarbitrage can review and optionally merge it into their PR.

[oxarbitrage]

thanks, trying it.