Add test call_migrate_works to VM

In `packages/vm/src/calls.rs` we have tests for `call_init`, `call_handle` and `call_query`. `call_migrate` is missing.

This can be added using the hackatom development contract, with some simple logic as here: https://github.com/CosmWasm/cosmwasm/blob/v0.13.1/contracts/hackatom/tests/integration.rs#L133-L139