Add hardhat config file checking

[FabijanC]

While integrating scarb, I used this in the run_test function of test.sh:

    npx hardhat --typecheck --config "$CONFIG_FILE_NAME" 1>/dev/null ||
        echo "Invalid config file!"

We need a better version of this:

• exit after reporting an invalid config file
• restore the state as it was before the test (or don't change it at all - I was changing it by replacing the config file because I couldn't find another way of validating it).