Feature: add Mock Contract Boilerplate for usage in library

🧐 Motivation

Common testing pattern in Solidity is to mock ERC tokens such as:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";

//Simple Mock ERC721
contract MockERC721 is ERC721 {
    constructor() ERC721("MockNFT", "MNFT") {}

    function mint(address to, uint256 tokenId) public {
        _mint(to, tokenId);
    }
}

This pattern is repeated over and over again, however the OZ Contracts do not include such trivial boilerplate contracts, in fact the contracts found in /mock are for internal testing and not typically used by end developers.

📝 Details

Include a *Mocking Harness (I use harness to distinguish between the existing usage of Mocks in the codebase and this new one meant to be used by end users

ERC20

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC20/ERC20.sol";

// A simple Mock ERC20 for testing purposes.
contract MockERC20 is ERC20 {
    constructor(string memory name, string memory symbol, uint256 initialSupply) ERC20(name, symbol) {
        _mint(msg.sender, initialSupply);
    }

    function mint(address to, uint256 amount) public {
        _mint(to, amount);
    }
}

ERC721

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";

// Simple Mock ERC721
contract MockERC721 is ERC721 {
    constructor() ERC721("MockNFT", "MNFT") {}

    function mint(address to, uint256 tokenId) public {
        _mint(to, tokenId);
    }
}

Naive Usage

[!NOTE]
The path is just an example, for this issue, change as you will!

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts/token/ERC20/ERC20.sol";

import "@openzeppelin/contracts/token/ERC721/IERC721.sol";
import "@openzeppelin/contracts/token/ERC721/ERC721.sol";

import "@openzeppelin/contracts/testing/token/MockERC20.sol";
import "@openzeppelin/contracts/testing/token/MockERC721.sol";

// etc etc

Conclusion

No more unneeded boilerplate copy paste yes plz sers?

I can open a PR if you would like,

Cheers!

Start by reviewing the existing contracts in /mock and the ERC20 and ERC721 entry points referenced in the issue. Compare their internal-testing role with the proposed end-user MockERC20 and MockERC721 harnesses, then determine the supported scope and package path; done means the agreed harness contracts are available through the documented import paths without duplicating internal mocks.