Create Basic Solidity Unit Tests

[jhelison]

Is your feature request related to a problem? Please describe.
Lack of test coverage means broken contract changes can be merged.

Describe the solution you'd like
Add sample unit tests in JavaScript under test/ for example contracts (e.g. test ERC20 mint and transfer).

Describe alternatives you've considered
Manual test execution only—lower visibility.

Additional context
Use Hardhat test framework.