Part 3 of https://github.com/orgs/solana-program/projects/8.

(diff is cumulative)

Problem

solana-program-test is on its slow way out.

Solution

Convert these tests to Mollusk, including simple but perfectly equivalent stake tracking.

This PR adds:

• DelegateConfig
• Delegate tests, migrated from program_test.rs to delegate.rs
• StakeTracker and a MolluskStakeExt trait which allows delegations to be tracking when warping across epochs,
• Extensive equivalence testing demonstrating that the new StakeTracker acts as BanksClient does when tracking delegations
• Final updates to helpers; the remaining PRs will be (almost) exclusively additional tests, not harness items

This does NOT migrate the tests from the newer stake_instruction.rs, which already uses Mollusk.