Expand TestHost with remaining host functions

[limemloh]

In the concordium-smart-contract-engine we provide a TestHost for running smart contracts locally. This is then used in cargo concordium test to run unit tests that are compiled to target WASM.
Currently, these unit tests can only rely on host functions related to the contract state, so this task would be to expand this with the remaining host-functions (some might not make sense to implement) allowing the smart contract user to mock these in a unit test.
This would probably also include adding a host function for setting a mocked environment.