Migrate smart contract tests from Hardhat to Foundry.

[ryley-o]

This is currently a watch-item, based on Foundry adoption and/or advantages.

Goal: This will improve developer efficiency, code sharing, reliability, and code health as a function of time. No longer as urgent after previous eng work (previously tests took long time but now run in parallel and no longer take a long time).

[ryley-o]

With the changes in #498, this becomes less valuable thanks to hardhat fixtures being much more performant than our previous use of mocha's beforeEach. We can keep this issue though, since using Foundry would enable our tests to be written in solidity, and should still be considered an option, even if we are less likely to pursue.