Use a standard ERC20 token instead of our unsafe non-standard version

[samlaf]

Currently we use a weird forked erc20 mock token that doesn't check allowance in transferFrom, and also mints on transfer I believe, just so that tests are easier to write.
Given that inc-sq is becoming more serious with more eyes on it, we shouldn't be doing this and should follow proper standards.

Let's use a standard openzeppelin ERC20 token standard and fix our tests to mint and allow where needed. See discussion in https://github.com/Layr-Labs/incredible-squaring-avs/pull/48/files#r1585342637

[0xpanicError]

Can I take this?