Migrate test framework to Hardhat, cont.

[kronosapiens]

Continuation of #1196 which upgrades the codebase to Ethers 6 and migrates to the use of an in-process node for testing

WIP

To do:

• Test reversions: https://hardhat.org/hardhat-chai-matchers/docs/reference#reverted-transactions
• Test events: https://hardhat.org/hardhat-chai-matchers/docs/reference#events
• Balance changes: https://hardhat.org/hardhat-chai-matchers/docs/reference#balance-change