Add tests for upgradeable contracts

[fadeev]

          i would suggest that we add in future tests for upgradable contracts, upgrading between 2 versions and establishing a process for upgrading using hardhat-upgrades

for example, also useful to keep in mind - i think when contract is deployed with hardhat upgrades, there is useful artifact for contract storage that should be kept in source control, so next time contract is upgraded this artifact is used by hardhat-upgrades plugin to verify if storage is broken and upgrade might not be valid

Originally posted by @skosito in #6 (comment)