chore(test): migrating tests from oz-test-env to hardhat

[fforbeck]

Proposed Changes

• Refactored all tests to work with Hardhat + Truffle5 plugin.
• Fixed test and code coverage workflows.
• Removed oz-test-env lib (no longer maintained by OZ team).
• Etherscan verification task still uses truffle to compile the contracts, it will be migrated to hardhat in another PR.

[codecov[bot]]

Codecov Report

Merging #520 (9d56a2d) into master (318d426) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 9d56a2d differs from pull request most recent head 8a75e7c. Consider uploading reports for the commit 8a75e7c to get more accurate results

@@           Coverage Diff           @@
##           master     #520   +/-   ##
=======================================
  Coverage   83.28%   83.28%           
=======================================
  Files          53       53           
  Lines        1855     1855           
  Branches      431      431           
=======================================
  Hits         1545     1545           
  Misses        310      310           

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