WIP: Tests/Sandbox: create an opam package for bin_sandbox (integration tests using flextesa)

Annoying in term of (opam section) CI as it adds a super computationally heavy test (the new package depends on both tezos-node and tezos-client which are the most expensive tests so far!) But mandatory on the road of releasing the code in opam as all tests have to be part of an opam package. (There is an opposite direction to follow: removing the runtest alias in the dune file ... but I don't think we want that.)