add(consensus): Add `slow_start_{interval/shift}` fields to `testnet::Parameters`

[arya2]

Motivation

This PR adds fields to testnet::Parameters and is cleanup for #8475.

Part of #7119.

Depends-On: #8475.

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Solution

• Adds slow_start_interval and slow_start_shift fields to testnet::Parameters
• Moves SLOW_START_INTERVAL and SLOW_START_SHIFT constants to zebra-chain

Testing

The new_regtest() method should be tested by regtest_submit_blocks(), some of the rest should be covered by existing tests, there may be a minor gap in coverage considering a mistake was made in the previous PR.

Review

Anyone can review.

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[upbqdn]

@arya, this PR needs a manual conflict resolution.