Write invariant testing for Heap.sol

[Amxx]

          I tried testing these invariants with [Foundry Invariant Testing](https://book.getfoundry.sh/forge/invariant-testing) without success. The reason is that we need a mock to deploy within the setup function so that it acts as the target contract to validate the invariant against.

I don't think that's needed but we may want to reconsider invariant tests

Originally posted by @ernestognw in https://github.com/OpenZeppelin/openzeppelin-contracts/pull/5084#discussion_r1687475372

[Amxx]

Ref: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/5084#discussion_r1687475978