add support for the label cheat

[bohendo]

Commonly used in foundry unit tests, adding support for this cheatcode will get us closer to being able to re-use existing setUp functions out-of-the-box, eg:

import { ExistingUnitTestBase } from "./ExistingUnitTestBase.sol";

contract MedusaHarness is ExistingUnitTestBase {
    function setUp() public virtual override {
        ExistingUnitTestBase.setUp();
    }

    // now fuzz fns & invariant checks have access to eg an example system deployment, helpful addresses/tokens, etc
}