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 #5084 (comment)

[Amxx]

Ref: #5084 (comment)